Thanks all for the advice. Dirk, good to know I can be fairly heavy handed with the heat, I'll try this first if I have to end up taking the plate off, which I think is actually my only option. If I do this however I will almost definitely have to remove the block which may well mean I have problems with other bolts.
Nasher, trip was great thanks but I missed the ribbing! I guess snowboarding in perfect powder and riding some of the best lift accessed mountain biking in the world made up for it a little
The welding nut thing is a good option if the mole grips don't work.