HT coil and lead and plug cap checked for resistance and within tolerances.
Cleaned and greased and reassembled.
Will check valve clearances tomorrow but I'd be surprised if it's them.
Only occurred from 4200rpm upwards and in both 4th and 5th.
Due a 6k service ie no valve clearances but I'll do them to be sure. Air filter was ok, a little dirty maybe but not terrible. Plug was in good condition.
Hmm.