Thanks for your comments Brian and terryk,
engine now fully dismantled, and I can see the problem. When the barrels were removed, the LH piston did not move on the gudeon pin.
When the crank came out, the bearing surfaces looked fine.
Looking up the con rod, the small end bearing looks like it has failed - that would be the lumps seen in the oil. There was a wee bit of play evident at the small end when I rebuilt the engine, but thought it was small enough to be ok, obviously not.
I have full faith in the engineer who did the bearing - don't know the details, but I think he would have sized the bearing up to match the crank, as he did my regrind. I'm sure he does a 'proper job'.
Will be calling on him Monday morning on the way to work, for his expertise in assessing how a repair can be undertaken. Hopefully it wil be just a replacement of the small end bearing.
Pictures attached to show you what I found in my oil that initially caused my concern, a shot attempting to show the underside of the piston (you can just make out something wrong between the top of the con rod and the piston), as well as the main bearing and crank bearing surface.
I feel much better now.
John