After spending several days trying to get hold of a flange puller that would fit my limited space. David of Indigo Yachts in Bassterre taught me a fine method as follows:
0. Liberally apply TD Blaster or your choice of penetrating oil and leave overnight
1. Get a socket that will fit over the gearbox shaft end but is smaller than the prop shaft
2. Get overlength bolts that can pull the two flanges together with the trapped socket between them. The bolts should be just long enough to load the nut. Too long will foul the gearbox case.
3. Apply some tefgel or other anti-siezing compound to the bolts, particularly if they are stainless
4. Gradually tighten the bolts to ease the flange off against the socket
5. When the bolts start to foul the gearbox case remove them and backfill with washers or oversized nuts to reduce the length
6. If the flanges close up before releasing get a second socket and align it with the first with a cardboard tube
7. Repeat this process until the flange comes free.

It took a while but this worked perfectly for me. Good enough that I will not carry a dedicated flange puller but I will secure the required bolts.