Thanks for all the responses gents...appreciated!
To answer all your suggestions. It’s difficult to tell if I increase speed slowly whether it makes a difference. It’s not obvious but I think it may do slightly. At this point a prop-shaft key failure would be a good outcome, but somehow I doubt that is the issue....but until I can dive on it and test this by hand, I won’t not be able to tell for sure. Though of course will need to do so. To answer another question, I have the standard prop. I did place my GoPro underwater and run the engine, and the prop does seem too increase in speed....though hard to tell if it’s slipping on camera.
Bad cable adjustment is what I thought lead to the first failure, hence me rebuilding the gearbox (Hurth HBW 250). I definitely didn’t have this right when I initially installed the new control cables as I lost reverse due to it not engaging properly and had to adjust it again. After that it worked fine for one outing and then I lost drive completely. After that I figured out I had probably burnt out the clutch, stripped the box and rebuilt it with the official rebuild kit, new thrust washers, plates, shims, seals, and I bought and installed a new bearing kit at the same time for good measure.
So I fully expected everything to work perfectly after the install. I carefully adjusted the cable this time to ensure proper engagement. When I failed to get drive again, I completely took the cable off and shifted by hand, still nothing.
I feel I did a good job of the gearbox rebuild, measuring free-play and backslap, and installing the correct shims, thrust washers and components, so I am fairly confident it now isn’t the gearbox...but who knows? Stumped...LOL.
I don’t think it is the c-drive, that is moving smoothly by hand as I think it should. So it comes back to the prop or the gearbox. I guess I’d better go and buy some dive gear and get down below on the prop! BTW in England the water is not welcoming, so I will need a wetsuit...unlike some of you guys in those lovely warm waters down south!