Hi All.
An endoscope inspection in situ of gear on transmission shaft revealed a blue gear (6 tooth?) appearing to be quite worn despite the poor resolution with the viewer. A comprehensive internet search revealed that no new ones seem to be available and used ones are VERY rare and difficult to obtain. Fitting a new 18 tooth DRIVEN gear has given me a properly functioning speedo for the moment (the engagement of the two gears must be quite small compared with new specifications). So OK for now. Perhaps this would be good project for a 3D printer working in nylon or acetal resin if the number of DRIVE gears required nationwide could justify the outlay. Just a thought . Anyway - Thanks to all for you contributions.
kirky2