Thanks to those that posted here, especially KH via mail.
It seems the problem has been solved and was a little nearer home than I was looking. A new rotor arm seems to have sorted the problem completely. No more falters/misfires after 10 mins or so - no more problems starting when hot. The arm I replaced wasn't too old however it does appear to have been the culprit. Why it started to cause problems when warm I dunno - anybody ?
Thanks folks.