An update: THE BASTARD IS OFF!
Pulling: didn't work.
Hanging: didn't work.
Prying: didn't work.
I ended up getting 4x 60mm grade 8 M6, 4x 70mm grade 8 M6 plus 8x M6 steel thread couplers and painstakingly using them as jacks to push the bloody thing off mm by mm. The last little bit was accomplished by cutting the heads off the longer bolts and using them to push 4x studs into the holes of the flange. A last minute jiggle with a wrecking bar and it was off. Nothing appears to be busted after a cursory look.
Verdict: the spline had locked itself onto the planet gear carrier. No signs of corrosion though.
I'll work on it during the week, replacing the remainder of the seals and gaskets, especially the leaking seal for the selector shaft.
Time for a beer.