Just spoke to BT on there "Online Chat" and someone on there was able to reset the SNR for me and he said it could take 24hrs so we will have to see. Thanks for your help.

The automatic system does work still, my line which has gone stable after the Christmas storms is back to sensible speeds once more.

Deliberately did not pester anyone to see if it did recover on its own - advantage of a second line on LLU

I still reckon there is remote interference affecting the circuit. That's erroring to quickly after the reset.