It's funny but, cars are similar to boats for fuel consumption where bigger/smaller engines are concerned and I don't mean they have similar fuel consumptions.
For example, Martin and James's boat with the bigger, well massive actually, brute of a V8

produces 300hp

but it can also be as economical as a smaller 5.0 l V8 at intermediate revs. Again, I'm no expert but what I see is that the 350 mag is an electronically controlled engine with masses of power but with very accurate fuel injection and fuel monitoring. Also put into a boat of the right size and weight it could out perform a smaller engine for not just its shere power but in economy.
I would have an absolute guess that with a 5.0l V8 at 1,000 revs in the 240 you would get roughly what Martins chart says about 5.5litres an hour and at 1500 revs about 10 litres an hour.
I'm guessing, but if you pottered about and didn't reach the higher figures you might get away with double the cost. Even at 1500 revs and 10l per hour, half a day at 5hrs for example would be 50l plus a bit of gas guzzling!!
You haven't said what you want to do with it for the other 9 months, not that the British weather will let you of course!