Switched to Talktalk Fibre on 14th December. Worked fine for a couple of days (sync speed 38mb, speed test contanst 20mb). Then it went off completely for about 12 hours. Next morning back on again. And it's been doing that randomly ever since, up and down, off for hours at a time usually.
When it's not working, the broadband and internet lights are both off (no occasional flashes or any sign of activity/attempting to sync). I've logged into the talktalk router and it says "no signal". Rebooting the router makes no difference, neither does unplugging the rj11 cable. I've pulled the faceplate off the BT socket and gone stright into the test socket with an external filter and that makes no difference either. There's no telephone or any other devices plugged in at any time (so I don't know if the phone is going off as well).
I'm out of ideas now, guess it must be either a faulty router or an external fault?