Well, since originally posting the car will now occasionally start running really rough - hunting at idle wildly, from stall to 2,000 revs like it's desparately trying to control itself. Sort of runs OK once you get going. Has usually rectified itself within a few minutes but recently it stayed like this. All sorts of codes, not just the misfires, O2 sensors, MAF sensor etc accompanied by CEL and, strangely. ESP lights.
Managed to limp to the main dealer as starting to get fed up with it. Guess what - they started it up and it was fine! Loads of codes though. They kept it for two days, said they spent hours on it, changed a £25 power relay (??) charged me £750 (ouch!) and it has been fine for a couple of weeks, then multiple misfires, blinking CEL and yesterday cough, splutter, really rough idle again.
Spent most of today pulling everything off the top, including rocker covers. Cleaned all connectors, crank breathers, checked vacuum lines, plugs etc etc.
Now drives and idles OK but still getting random misfires on one bank (strangely only 4 & 6 now)