I doubt that TBH, if your confident that it's been getting a good +12V from the other battery (which I'm sure it was) my money would be on the solenoid part of the starter, is there anyway you can give it a knock and at the same time apply a good charge to it, that should help to free it off. I realise you've tried this but there can't be much more of an explanation apart from something more serious like a seized or locked engine.
I'm going to have another go at it later mate, i might try and crank the engine by hand and see if it moves, she was running just fine when i removed it 18months ago and has sat in my garage nice and dry in the mean time, do cossie blocks often seize up when stood?