Just given them a call and finally all sorted. Profile needed manually resetting to 78 meg.

Oddly after 5 BT engineer visits they've changed so much kit and reconnected so many joints that they have no idea where the fault was. They suspect it was a faulty faceplate on the master socket. But that was changed on the 2nd engineer visit last Saturday. Ahh well, I don't care what it was, just pleased its fixed. Hopefully it doesn't come back.

Big thanks for all the advice on here. Especially big thank you to the Plusnet staff on here for keeping it pushed along and helping.

Cheers
Stu

Having a similar issue. Just raised a ticket. Line reports a fault.