Your line should be doing 60Mbps and you're getting 18. The router lights look fine. Your ISP says the line is "synced normally." But every video call freezes and the kids are shouting from upstairs again.
Nine times out of ten, it's not your ISP. It's something inside your building between the green cabinet on the street and the back of your router. Here are the nine causes I see most often in London flats, in rough order of how common they are.
1. Old internal wiring (bell wire from the '80s)
Open the master socket faceplate in any London flat over twenty years old and there's a fair chance you'll find what we call "bell wire" running off to an old extension — thin, untwisted, often a single pair of solid copper insulated in PVC. Fine for ringing a phone bell in 1984. Terrible for VDSL broadband in 2026.
The fix: disconnect all old extension wiring at the master socket, then test. If speed jumps, the wiring is the problem. The proper repair is to pull the old stuff out and re-run as Cat5e or Cat6 — or, if you only use one socket for the router, just leave the extensions disconnected.
2. No filtered faceplate on the master socket
If you've still got an NTE5 (not the newer NTE5c), broadband is being filtered with separate microfilters at every extension. That's how it was done in 2002. It's not how it's done now. Anyone of any vintage on VDSL or FTTC should have a filtered faceplate — an NTE5c — which moves the filter inside the master socket itself.
The fix: swap the front plate for an NTE5c. About 15 minutes of work. Usually picks up 4–8Mbps on a noisy line. It's the single most under-appreciated upgrade in the whole telephone trade.
3. Long extension runs degrading the signal
Master socket is by the front door. Router is in the bedroom at the back of the flat, 12 metres away, daisy-chained through two extensions. Every connection point introduces resistance and reflection — your broadband sync speed at the router is meaningfully lower than at the master.
The fix: move the router to live next to the master socket, then run Cat6 (not phone extension wire — actual Cat6) to wherever you want to use it. Use the router's ethernet ports + a WiFi access point at the far end. Phone-extension wire is the wrong cable for the job; we just used it for decades because nobody knew better.
4. Corroded connections in a damp riser cupboard
This is the classic London flat problem and the one tenants almost never see because the wiring isn't inside their flat. It's in the communal riser — the vertical cable cupboard that runs floor-to-floor in mansion blocks and converted houses.
Old buildings have damp risers. Damp risers corrode terminals. Corrosion adds resistance to your pair. Your broadband line "tests fine from the exchange" because at low power the connection still works, but at the higher signalling frequencies VDSL uses, it's a mess.
Aisha in Camden had a ground-floor flat in a 1930s mansion block. Sky said her line was fine. Her sync speed was 18Mbps on a line that should've been doing 50+ on FTTC. I opened the riser cupboard in the communal hallway and found six flats' worth of phone wiring tee'd off a single pair with scotchlok connectors from the 1990s. Three of the connections were green with corrosion. I re-terminated her pair properly on a Krone strip, ran a fresh cable from the riser into her flat, and she synced at 48Mbps that afternoon.
The other five flats are probably still on 18. Half the building doesn't know they're being throttled by 30-year-old scotchloks in a cupboard nobody's opened since '94.
The fix: not a DIY one. Get an engineer in to open the riser, find your pair, re-terminate on a Krone strip, and run a fresh cable to your flat. If you're a leaseholder, the managing agent has to give us access. If you're renting, your landlord needs to authorise it.
5. Shared riser cabling in mansion blocks
Closely related to (4) but worth its own entry. A lot of London mansion blocks have one or two phone pairs serving multiple flats by historical accident — the original GPO install was for one resident, the building got subdivided, and successive engineers tee'd new flats off the existing pair instead of running fresh cabling from the head.
The result: when your neighbour's broadband is busy, your line is busy too. They share electrical noise. They share faults.
The fix: a proper pair audit by an engineer (we trace each flat back to the building's distribution point), then a re-cable so every flat is on its own dedicated pair. Bigger job — usually a half-day across the whole block — but it solves the problem permanently for everyone.
6. Router placed on top of the microwave / next to the TV
Microwaves run at 2.4GHz. So does the older WiFi band. So does your baby monitor and your wireless doorbell. Put the router two feet from a running microwave and the WiFi signal will drop every time someone reheats coffee.
The TV is less obvious but worse: modern TVs have powerful WiFi receivers built in, plus a HDMI port pumping out radio noise from the HDCP signalling, plus they sit at the same height as the router antennae, plus they're often inside a big metal enclosure that reflects signals all over the room.
The fix: the router goes up high, in the open, away from other electronics. Top of a bookshelf, on the wall, on a small high shelf. Not behind the TV. Not on the microwave. Free fix, takes 30 seconds, fixes more "broadband problems" than people realise.
7. Aluminium foil-backed insulation blocking WiFi
Newer London flats, especially those in 2010s-2020s conversions, often have foil-backed plasterboard on internal walls for thermal/acoustic insulation. The foil is a near-perfect WiFi shield — radio waves don't go through aluminium foil any more than they go through a Faraday cage.
You'll spot it because the WiFi is fine in the room with the router and drops a cliff edge as soon as you cross the wall. Not a gentle degradation; an edge.
The fix: mesh systems won't fully fix this — they help, but you still need a node in every room that can see the next one. The proper solution is hardwired access points: Cat6 to a few wall-mounted APs, kill the router's own WiFi, let the APs do the radio. More work upfront, but it's the only thing that genuinely works.
8. REIN (Repetitive Electrical Impulse Noise)
This one is brilliant and frustrating in equal measure. Certain household electronics pump electrical noise back through their mains lead, into the building's wiring, and from there into your phone pair if it's running near the mains cable. The most common culprits in London right now: cheap LED lightbulbs, LED dimmer switches, phone chargers, electric blinds, fish-tank heaters.
The symptom is always the same: broadband works fine, then drops out repeatedly at the same time of day. Evening, usually, because that's when people turn lights on.
Sarah in Stoke Newington rang me on a Tuesday — her broadband had been dropping out every evening around 7pm for three weeks. Sky told her the line was fine. I turned up, plugged my test kit into the master socket, and the SNR margin was jumping all over the place between 9pm and 11pm. Classic REIN signature.
Turned out her downstairs neighbour had installed a cheap LED dimmer switch that was pumping electrical noise straight into the phone line through the shared riser. Took me 20 minutes to fit a filtered faceplate at her end and an inline VDSL filter on the riser side. Sorted. She'd been on hold with Sky for longer than the actual fix took.
The fix: diagnostic — an engineer plugs in a noise meter at the master socket and watches the SNR margin. If REIN is the cause, the meter will pick it up within a few minutes. Then it's about isolating which appliance: unplug things one at a time. Once we know what it is, the fix is usually a small ferrite choke on the offending appliance's mains lead, or moving the offender further from the phone wiring.
9. The line is fine — it's the ISP
Worth saying out loud: sometimes none of the above applies, and the problem really is just your ISP. Contention ratios are high. Their backhaul is congested. You're paying for 80Mbps and you're getting 30 because their network is over-sold in your area.
You can spot this with a simple test: take a laptop, run an ethernet cable directly into the back of the router, and run a speed test. If you're getting close to your headline speed wired, the bottleneck is WiFi or the rest of the wiring. If you're getting nowhere near it wired, the bottleneck is the line or the ISP.
The fix: switch providers. Or — if FTTP is available in your area and you're on FTTC — upgrade to fibre. That fixes about 80% of cases where the line itself is the bottleneck. Most of London has FTTP now; check Openreach's availability map.
Marcus in Greenwich is a related but different problem. New-build flat, two years old. The developer had put the master socket and the Virgin Media ONT in the same cupboard — a cupboard with no ventilation and no power socket. The router was overheating and rebooting every few hours. He thought his line was dropping. It was — but not because of anything outside the flat.
I moved the master socket to the living room wall (short run, easy in a new-build with stud walls), and fitted a small shelf with a ventilated enclosure for the router. His broadband hasn't dropped once in the three months since, last time he checked.
Always check the boring stuff first: is the router cool? In an open space? Plugged into the right port?
The five-minute self-diagnostic
Before you call an engineer (or yell at your ISP), do this:
- Open the master socket faceplate. Behind it is a "test socket" — a single port that bypasses all your internal wiring. Plug the router directly into that. Reboot it. Wait 5 minutes.
- Run a wired speed test from a laptop plugged into the router with an ethernet cable.
- If speed is good at the test socket, the problem is in your internal wiring or your normal socket. Internal wiring is our patch.
- If speed is still bad at the test socket, the problem is the line or the ISP. Ring the ISP first — they should raise an Openreach ticket.
- If wired speed is fine but WiFi speed is bad — the problem is WiFi coverage, not broadband. That's a different fix entirely.
Common questions
How do I know if it's my line or my ISP?
The test-socket diagnostic above. Plug the router into the test socket inside the master socket, run a wired speed test, see if the line delivers what the ISP says it should. If yes, the rest of the building is the problem. If no, the line or the ISP is.
Does the age of the building really affect broadband speed?
Yes. Older properties have older internal wiring, more daisy-chained extensions, and more shared riser cabling in communal areas. All of those drop the line's sync speed below what the exchange can deliver. A 1990s flat will almost always sync faster than a Victorian conversion on the same street.
Can I fix slow broadband myself?
Some of it. Moving the router away from interference takes 30 seconds. Plugging the router straight into the test socket diagnoses whether the problem is internal. Anything inside the walls, in a riser cupboard or out at the cabinet is engineer work.
Slow line you can't pin down?
Ring me with the postcode, the property type (Victorian terrace, mansion block, new-build, etc) and what your current sync speed is. 90 seconds on the phone is usually enough to know which of these nine it is.