Well, sussed one problem, I think. A friend had stood on the cables while tending the rope for a bit of impromptu barefoot water skiing (failed), and they weren't leading properly. This put an upwards pull on the linkage causing it to jam occasionally. Just moving the cables sorted this.
Sadly, the gearbox is stuck in forward gear now. The shift shaft on the box will not go into reverse, but will find neutral, though it reverts back to forward as soon as I turn the drive shaft. Something's obviously not right, just wish I knew what it was. Trouble is, I don't really understand these gearboxes.
Today I tried to get the bearing carrier out with no success (10 years old and never been removed). Have given it to the yacht centre, should get a report tomorrow. Guess this is going to cost close to a grand by the time I'm finished - judging by the prices on the Mercury Parts web site.