Seems there wasn't that much amiss, which really surprised me as after 30 year of bike riding I'd convinced myself it sounded seriously unwell.
Problems found included a slipping aux belt (which I was partially aware of) and badly unbalanced throttle bodies!
other than this, which has now been rectified, Steve seemed to think the engine was a good one.
I can only put my paranoia down to fact that bike had been standing for a few weeks (normally ridden daily) and that I was riding between a row of terraced shops, possibly without earplugs. Tho I do sometimes deliberately not use these on a short ride so as to 'listen' to and check the mechanical note anyway.
Neal