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just to complete this. After my last call to BT technical support the following day an engineer turned up at the door, I wasn't even aware that an engineer had been booked. After a chat with the engineer (boost) he couldn't find any problems, yet it dropped ppp while he was here. He spoke to control who advised there was crc errors building up on the line, yet there was no crc errors at my end with my equipment or his JDSU, so he went down to the cab and swapped out e-side with a spare pair and a reset of the card in the exchange, finally I now have a stable connection and it has been connected for 5 days, last thing to get done is an SNR reset.
This was strange as there was no noise on the line, no crc, fec or header errors, yet the ppp session dropped while it stayed in sync, most likely the card reset did the trick.