Made a puller and got the flywheel off. Thanks for the help.
Got at my snapped camchain which had dropped down behind the flywheel.
I bought this engine from a breaker, didn't fit it while it was still under the weeks warranty, left it ticking over and it just stopped.
I thought that maybe the engine had been stood for ages and the camchain had gone rusty and snapped. But on inspection, and because the camchain tensioner screws were not done up tight enough, I now think that the cam chain tensioner had been removed - then screwed back in without setting the tension correctly, so it was pressing in far too much.
Chain

Rear guide with (strange) small round push marks above where the tensioner locates

And rear guide showing excess wear
