I wont keep you all in suspenders any further ........................it was............................!!!!!!!!!!!!!
The bolts that attach the flywheel to a flange on the crankshaft had unwound themselves yes all of them! eventually they contacted the bell housing, ie clunk-stop.
The rebuilders reckon they weren't properly torqued at the factory, no harm to the engine , they had taken the head off as a precaution but nothing wrong there.
I just have to source a few gaskets for them and its all go , hopefully the bill won't be too steep for that, they said its a very common engi