Valve clearances checked and although slightly tight can wait until winter when I'll have plenty of time to have the cams out.
I'm currently the middle of tapping the AIS holes on the exhaust to block them off.
Next question....
Having drained around a gallon and half of 'rusty petrol' out of the tank, I'm left with quite a lot of debris in the bottom - any suggestions on how to get the majority of this out? I'm loath to use a sealer as the tank isn't holed or anything (and it'll gum up the level sensor). Did wonder if washing it out in diesel would work - it's heavier than petrol, may hold the debris in suspension to drain it out?
After this my attention turns to the carbs - guessing these will have ingested rusty petrol!
I'm feeling fairly happy now that I know why it doesn't run, and it should be good once the servicing/cleaning is complete.