Jimmy - Thanks for the response, much appreciated.... I figured the two boxes under the tank must be CDIs but then there's another one under the seat unit so there seems to be three on the bike?

Your post suggests that swapping the two CDIs won't cause any damage so I'm going to try that over the weekend to make sure that one of them is knackered. I'll then try taking the top off and seeing whether anything obvious is amiss.

Thanks again...