well that was a PITA to find this problem but think ive finally found the cause. the bit of pipe that joins the 2 exhaust manifolds together is leaking so one can assume that the lambda is picking up more oxygen in the exhaust , so that is telling the ecu to dump more fuel in to compensate the extra oxygen, hence the lean misfire when warm.just need to try seal it at the weekend ? any ideas on how I could seal the little nuts on the exhaust manifold the the joining pipes go into? cheers