Phoned BT again, I think more "tests" are going to be done and hopefully something might happen, said it would be good to get the phsyical line swapped over. Stats from my ISP are 16 disconnects in the last 24 hours, running at 5mbit down 300K up for stability.
As an update, I finally got another engineer out who could go to the exchange, after once again testing everything at my end he found a damaged wire in the exchange that was causing the problem and fixed it. Everything now is fine, with good clear line for phone and internet. It could have saved a lot of time and money if the origional engineer could have just checked that though (all 3 seemed to be BT openreach engineers, but the last one had come out at the request of the ISP)
So for anyone getting issues like this, with no fault found but a persistant random issue, chances are it is going to be the exchange end.