Once you have carried out the test as shown in the video and have ruled out any problems with the internal wiring, contact Talktalk fault reporting when they answer ask to be connected to the fault management team. hopefully they will be able to resolve the issue for you.
Thanks for the link. I've tested the connection directly to the test socket and there's no difference. Same speed and same random dropouts.
I think we might have been speaking to the wrong dept before, I didn't realise they had a specific fault department we were just ringing a generic 0345 number. But a little googling tells me that there is a number specifically for fault reports so we'll give that a try tomorrow when we're back at her house, assuming they've not got in touch before that.
My main gripe is they're just not keeping promises. They've been saying since early last week someone would get in touch within 24-48 hours and still now no-one has done so. Every time we ring in they just say they've escalated it to their management and that someone from management will get in touch. But again they don't. I appreciate BT might be busy but to be honest I just get the feeling TalkTalk are giving us the runaround and given the complete lack of concrete info I have no confidence that they've done anything to actually push the issue along.
I did try raising the issue with the team on their twitter feed but again they were of little help. Said they'd raised the issue and we could expect a text update "soon". When I pushed for a timescale again I was told up to 48 hours.
Anyway will keep on plugging away and thanks for the ideas folks.