well. just for future reference if anyone else needs help with simlar probs...
fitted a new set of correct plugs today, reset ecu, all seemed good, until a few miles down the road, then it started to miss and got progressively worse as before. i found the only way to get it to run better was to unplug the lambda sensors which are in the collector on the cast exhaust manifold.
i noticed when a changed the plugs the rear 3 were rich and the front 3 not so much. i plugged in the OBDII scanner and noticed the fuel trims were waaay out of whack! then i twigged i'd been a dumbass and plugged the 2 lambdas into the wrong plugs, so ecu thought the rear 3 were going lean and adding fuel when it was the front 3 going lean, and the problem just got worse as the ecu tried to correct itself!
touch wood, alls ok now and it runs lovely. will get some miles on it then start tuning!