Thanks again for input chaps. Different time zones mean I'm often snoring while you guys are posting!
@Olhogrider - I had thought of the exhaust, particularly as there's been more than usual backfiring lately, although having checked those nuts at the weekend as part of the routine I'd dsimissed it. It doesn't quite sound right for that to be the cause - it's more tappy, but I know these things can fool us so I will check it out.
@ Singh - Plenty to think about there, and I will start taking bits off and investigating. The oil is less than 2000 miles old, but did sit in the bike all winter. As I said, I was about to do a thorough 7500 mile service anyway and perhaps this will reveal something - particularly as I have the 18 tooth sprocket arriving today so will have that side cover off soon! I will also check the cam gears backlash while I'm in there. I like your additive idea too - I'll see if I can find something to flush through.
@ Scott - I had tried that listening trick, but with a screwdriver. Wooden spoon is a good idea, it'll resonate better. With the screwdriver I couldn't work out whether the sound was from the top front or lower down behind the square plate on the right. Hopefully the spoon will work better.
I have to be out today but will make a start this evening and carry on tomorrow.
Thanks again all for very helpful input.
Will