I finally got round to opening up the engine and inspecting the water channels. They are all pretty clear so I'm satisfied there is no problem with blockages or anything wrong with the water supply. I have, however, found a burnt deposit on the middle piston side wall, the section that is exposed through the exhaust port when the cylinder is at TDC. It doesn't apear to be a blow by problem, as the piston rings are clean in good condition and there is no fouling on the cylinder side or between the rings. I have also checked the compression and it is pretty much 100psi on all cylinders. At a guess I'd say the temperature in the exhause manifold was getting too high and burning the oil film on the middle cylinder, but that could be rubbish. Does anybody have any ideas? Unfortunately I'm quite a way from any outboard repair places and would like to try to diagnose the issue at least before I take the boat anywhere.