Managed to get around to trying to free up the waste gate yesterday evening. Took a while but after a little persuasion, and a lot of WD40, it was moving as it should so took it for a test drive and it would appear that that the misfire at high revs in 3rd and 4th had gone. Driven it into work this morning and still no misfire so (fingers crossed, touch wood, kiss a black cat, rub a chimney sweep over my body) I'm hoping I've solved my problem.
Thanks for all the input and advice, all truly appreciated and gratefully received.