Thanks for your input guys. I had a quick play this afternoon between Christmas shopping and other domestic tasks. None of the cylinders had as much compression as I would have expected, a teaspoon of oil had the pressures back up to about 150-175 on all but 3 pots.
I think I have to get the heads off and remove each piston for a proper look to see what is going on in there. I think it will be a long job but worth it in the end, I just hope I don't end up having to get new rings / liners or I may just have to source another decent engine, which could work out cheaper
Thanks again though, it's always good to get a second opinion