Well I had it to bits now and I couldn't see anything jamming it up. I have no idea if there was something by the flywheel but there was no way to see in there really. So I took the timing cover & belt off and there was nothing there. Then took the inlet & exhausts manifolds and finally the head. I then tried turning the head and it cranked on its own fine with no resistance. I then tried to crank the bottom and it still wouldn't budge indicating it was the bottom end that had seized up
I got the head off and onto the bench and couldn't see anything that looked different like gaps in the followers looked ok with no gaps and the valves on the bottom looked ok. I then turned my attention to the block and as expected there was about 3mm of oil sitting in each cylinder and at first it wouldn't crank by hand but after a second or two of pressure trying to turn the pulley with a socket it started to move! Slowly it got easier till eventually it would turn round without much resistance. I guess the inside of the cylinders are nicely oiled now although it doesn't look like any of the oil had seeped past the pistons into the sump?