so we cleaned the commutator carefully and then re-assembled it and it fired up and ran fine, just in time for the end of a sunny bank holiday! Slightly odd that it decided to suddenly stop working properly.
Anyway, thanks all for your help, an interesting learning curve!
i had the same problems and found it was the brushes on the starter motor, looked ok but got a new set from luptons in newport ( of pan est ) been great ever since. hope this helps
