I worked out that a few sticks of dowel fit in the holes of the flange. With them in a vice and the hub on the other end there was just enough leverage to loosen the cog with a chain whip. So, no need to build the wheel up again
However getting the flanges off is proofing a little more difficult. I soaked them in boiling hot water as you suggested to hopefully loosen them up but.. no good, can't seem to get em loose. I don't want to be to ruff with them just in case I bend the centres..
Anyone else got any suggestions? Is there something I could spray in them with? Or a more heavy duty solution I could soak them in without damaging which might loosen them up??