Or you could go the cheap route and just by a tarpaulin. I think I paid about £40 for one to cover my 6.3 RIB. Go for the top grade and it should last about 4 seasons. Disadvantage is it takes longer to fit and you can't tow with it! A lot cheaper though!
I'm not sure about covering the engine though. There is an argument that you should leave that uncovered and free to let the air circulate. Otherwise, over winter it will never dry out!
Graeme, I'd be interested how much your cover cost and where from. Can you post details or PM me? It would be nice to have a 'proper' one if not too expensive!