I put this down to the fact that a service was long overdue. So I finally got some pennies together and got the oil/filter and a new set of plugs on the old girl!
PROBLEM - now it only runs for a mile or so before one of the cylinders stops and it came to a complete stop a couple of times trying to get it home!
Not being particularly hot on engines I pulled the lead off each cylinder - stuck one of the old plugs in and tested both sparks to earth. The front seemed quite powerful compared to the rear so I am suspecting the rear being the problem. (it actually started just on the front cylinder from cold when testing the rear!)
The spark seemed poor from the rear and when I went back to check it again it seemed better. The bike then started and as I said before ran fine for a couple of miles before really pissing me off!

All I can think is that it could be the coil on the rear that was intermittently playing up and has got worse due to disturbing it whilst changing the plugs! I also covered all the contacts in silicon spray to try and get better contacts.
Is there a way of testing the feeds to the coils? I looked at the manual but come away a little confused and daunted. I'm happy to use a multimeter but would like a clear instruction before "stuffing my probes in there"

Help me Obi wan!