Ian,
I said I'd be right behind you! I managed to remove my inlet manifold OK today - see my "this does not sound good" thread.
Normally, silicone sealant is used just at the front and the rear - so if you cut through this you should be OK. The front and rear seals can be either cork or neoprene - but in my case just silicone sealant was used, without a seal. Mike Huddart has advised me to do the same on reassembly.
A modelling knife worked for me - and this should also cut through a cork or neoprene seal OK. If you've already done this, then it sounds as though silicone sealant may possibly also have been used on two sides - and as the gaskets are quite thin, it is difficult to get even a knife blade in there.
Had I been in the same situation, my next step was going to be to drill a couple of holes in a long metal bar and to place bolts through to the carb stud holes - then to "wobble" the manifold from side to side using the leverage from the metal bar. But I was pleasantly surprised (and relieved!) when it came off just by a small prise with a blunt chisel.
Good luck - I hope you get it off OK, and apologies for my previous duff advice re levering on the thermostat housing
