Wot no reply?! If you haven't sorted it yet, I'd guess you're using a punch slightly too small or tapered? There are (should be) 2 pins, one inside the other.
That is only correct if they are original Graham. They stopped fitting 2 pins and replaced them with one roll pin later. You cannot get the 2 different size pins today, only the single one - I still have some here as I bought a packet not that long ago for two that I worked on.
If you can't get a simply roll pin out even with an air hammer, Mike, I would suggest there is something drastically wrong - such as there is no roll pin and you are knocking against something solid. Split roll pins can never be that tight by the virtue of the split along its length.
Are you sure the drive shaft is actually fixed to the gearbox shaft by a pin? Since it could never come off on it's own (there is insufficient space) I'm wondering if the last person who had it apart, put the shaft back on the wrong splines so the holes didn't line up and therefore no pin could be used, and rather than strip it all again when he realised his mistake, decided just to leave it without any pin.
When I put the ones I was working on back together, I found that the holes only line up in one place - they won't line up even at 180 degrees around, as the hole is slightly off centre.
Roy