Well I managed to remove the bl...y thing at last,
it's the most nerve wracking job I've had in many years. In the end I had to strip the unit down and then use the vice together with a couple of grip blocks. I still had to use a hammer and wooden block, with each blow 'ping' and a nail would fly out. This happened so often it drove me up the wall across the ceiling and down the other side,
so in the end I removed the nails and made some plastic wedges, and this worked, but it still took a lot of hammering and rotating the unit before the gearbox came out.
The cause of the problem was at some time or other a load of very sticky gunge had been poured over part of the motor and most of the gearbox, I had a hard job to clean this off when I removed the motor and didn't think about it anymore. This gunge had penetrated into the gap between the gearbox and the finger assembly, I assume with time it had started to harden and make a very efficient seal.
Anyway the gearbox is now clean, thank goodness, and awaiting greasing and reassembly.