Head removed, found one difference between top cylinders and bottom 2, small lip/ridge on the bottom (6 O'CLock) of the cylinder right at the top (last 5mm). I assume this is the limit of the piston ring.
Some play across the pistons, but it's uniform across them all so i assume this is normal.
Unsure where to go next, thinking of replacing the O-rings and seeing if that makes a difference, failing that i'm guessing rings or cylinder is warped? (will need a bore gauge)
Incidentally most of the small waterways appear to be blocked with salt and calcium build up.
Only other noticeable difference was a large build up of oil in the two bottom cylinders in the ports (exhaust i think, ports closest to centre of engine).
Anyone know much about changing the rings on these, rings appear about £20, but unsure how many more consumables i need to change in the process (bearings/shims etc)?