I had changed plugs and one coil, which fixed the misfire. But this is less pronounced problem, just a stumble, not enough to throw misfire codes. Some threads on US forum seem to recommend running at 4.5k rpms in 4th for 20 mins to burn some of the buildup and seems to have a short term benefit, but not long term fix.
The funny thing is the stumble is only between 3-4k rpms in 5th & 6th at WOT.
Probably no coincidence that I have been pootling round in 6th for a while and not being able to give the engine a proper run out very often.