hello mark,
about 2 months ago my car was firing on 5.
it wasn't the ptu (swapped with another)
it wasn't the coilpacks (interchanged)
it wasn't the coilpack or injector connectors even though they were green as the hulk before i cleaned them with an electrical solvent-amberclense/FE10 :mad:
it wasn't the spark plugs either.
unfortunately for me my no. 4 injector failed so it was over to the zen garage, plenum off, new gaskets, new bypass pipes, new injector. running sweet now :)
it cost me a fair amount of dosh mainly for labour as i didn't fancy taking the plenum off myself and then realising i had no way of getting the new seals/pipes/gaskets that would and did break.
i hope it isn't the injector as this is the most expensive outcome. but don't count it out like i did!
hth