Right - the rings were spaced and placed OK but when I buttoned it all up I went to find the almighty "Z" and I couldn't turn it over. I thought maybe the kickstart gear was being a dick again so I tried a couple of other things and yup, it was stuck.
Well I spent the better part of the night trying to think of what it was but no luck so I started taking it apart again in the reverse order. As you can see from the photos, the clutch and flywheel are fine, I didn't leave any rags inside the crankcase so I had the transmission off, the heads and barrels off along with all the strap on stuff that goes with them and it was down to the alternator stator. I put in a new rotor due to poor charging so the stator had to come off as well. When I looked at how the stator was siting on the engine case it looked a little off kilter to me so I loosened off the three screws that held it in place and bingo! the engine was turning over freely again. I'd been thinking the bearings were seizing up due to the salt air here and all kinds of other scenarios when I should have checked the simplest answer first. The clearance between the rotor and the stator is minimal and any deviation of the stator means contact with the coils and then she's locked up.
So the moral of this story is to look for simplest answer to the problems before delving into the heavy duty stuff. Oh well, I'm getting better at putting the transmission in myself and I remembered where everything went this time so only a day lost.