Bit over two days into the new line. So far looking good. Initially sync'd at 5200 and 6dB, just about made it through the evening drop but errors were racking up and SN margin was hovering around 2dB, generally my Speedtouch drops at about 1dB. Rebooted at start of noisy period, sync'd at 5000, dropped about 3.5 - 4dB during evening period but sync held, errors not too bad. Rebooted again farther in to noisy period at 4600 so far dropped 2.5-3 dB, which should hold with acceptable errors. That will give me an IP profile of 4000 which I'm happy with, actually I'm already at it thanks to some nice BT man. Overall I'm happy, stilll time to fall flat on my face but I'm optimistic. For the record my router is showing an attenuation of 54dB, please note Plusnet. In general it is looking marginally better than my previous connection prior to the fault.
So eventually got back to where I was, but took a month plus two goes at the high level complaints team. Have to say that once BT took it seriously they did a good solid job which took them well over the 2 hour job limit. Not that I'm complaining about that but it does point to the limitations of the 2 hour job limit. I still strongly feel that BT after a line fault should aim to return your line to approximately the same performance level for voice and DSL as it was prior to the fault as a matter of policy.
Thanks to everyone for the sympathy and suggestions.