Just as an update, I've still not managed to get the drive gear off it's taper. I work for a company that specialises in bearings, and we often have to extract them in various states and conditions from shafts of many designs and age. When I explained this non-work problem to one of my experts, he were amazed that such a design was intended to work after the warranty period was over. He was right, it doesn't, at least in my case.
I've tried most things now, from penetrating oil, to heat, to drifts, hammers and various pullers. I'm keeping restrained in the force that I use in case 65 year old metal succumbs to too much brute force.
The engine comes out of the frame next weekend, when I have another pair of hands, with magneto still attached. I plan to take it all to SRM for a strip and inspection so I hope and expect that they will have seen this particular problem before and will have the right tools and techniques to get the b****r off!
All the best, Simon