This vehicle when checked had nothing at the "D+" terminal with the alternator plugged in, with the alternator disconnected we had 2-3v, as a genuine alternator was only £20 more than the aftermarket i changed it and everything came back to normal.

i've taken all your comments on board and am waiting for another known good vehicle to carry out some tests to see if they are the same as the vehicle i had in...

Thanks for all your replies

Kris