Thank you for the recommendations, it sounds like either Celotex or 3M Thinsulate may be the best options and thanks smileypete for the idea to look on ebay for surplus or offcuts I hadn't thought of that. I had naively assumed that shops like B&Q wouldn't sell insulation that could be particularly flammable but of course the discussion above show that clearly isn't the case...Although I think that if a fire ever reached the boat bedroom and I was in it I'd already be screwed (as there is no exit), something less flammable may give me time to grab the engine room fire extinguisher and get to an exit. Hopefully the fire alarm would alert me before any of that was necessary however!
Regarding vapour barriers I now realise this may be necessary to stop moist air getting in or behind the insulation and creating condensation and rot, I will look into Duponts -thanks for the product suggestion. I also realise that this means sufficiently thick/good insulation to stop condensation occurring on the vapour barrier. We're not very keen on carpeting (previous owner had carpet which turned out to be full of rotten and moth ridden and was a nightmare to get off) and the wood has already been oiled in preparation for the walls so hopefully that should all be sufficient.
After analysing our condensation problem we've realised that it is mainly due to cooking, even though we have the windows open etc. the cooking vapour seems to make its way to a particular corner of the bedroom (the coldest surfaces in the boat). So I think putting up a curtain between kitchen and bedroom may help further prevent the problem.