Shanegtr - you were right on mate
tested today and plugged the trailer onto the D4 - turned on the lights and viola the tvan brakes locked up, did it a few times just in our driveway and confirmed this is the case. when I turn off the lights the tvan brakes work normally. shanegtr you were on the money mate well done. thanks to all other suggestions as well.
Prob easiest quick fix for now would be to disconnect no5 pin from the tvan plug till I sort out something more permanent. cheers everyone. [bigsmile]