Thanks for the replies everyone. I took it into the Mariner dealers today and waited around for a bit. At 1st they couldn't find any faults, apart from the fact that it sounded dog rough and didn't want to idle! The computer that they plugged in wasn't showing any current faults, just some in the history about the overheat and fuel problem. When I left them they were checking the plugs one by one. They all looked a bit coked up, black and wet.
I rang at about 4 this afternoon to see what the latest was and they have now got it running, quote "as sweet as a nut". They found the oil line wasn't connected properly!!?? I can only think that when I took the oil tank out from under the console to fill it up I disconnected something! Don't say it.
They are now checking the fuel tank sensor as this is not showing anything in the tank. They have said they want to get it out on the water to test it is OK at higher revs than they can manage on the mufs, but hopefully it is OK.
So basically, it was me.
I'll let you know the outcome of the sea trails, probably middle of next week as the boss of teh Mariner Dealers is going to the boat show.