FWIW, problem now solved...

After exhausted all my and the forum ideas I called Chris (pinball medic) who luckily lives near to me, he came around last night with schematics and worked out it was two of the chips on the driver board that were connected to a particular row on the play-field that were playing up. Was triggering targets as well as making the pop bumper coil go nuts..

Chips switched out and all perfect now.. Seems there was previously a lot of work done on the board, more than he had seen on a p2k board before, but all cleaned up and working now.