Ian,
Douglas timing pinions are traditionally designed to fit close up against the back wall of the timing chest, and not much bigger than the crankshaft shoulder they press up against. Consequently a special puller is required to remove them, particularly as the are often fitted ridiculously tight. Possibility you have tried a pair of screwdrivers and some heat to walk the pinion off and found it will not budge! The picture below is a heat-treated pinion puller for the DT/SW model engines, where the clearance behind the pinion is very minimal. But you get the general idea. For single use, you could skip the heat-treatment, but as the lip of the puller is very thin, it can use all the strength it can get.
-Doug
Larger view here
