From looking at the engine again, the big end bush is in 3 pieces - the inside surface is partially plated with steel from the crankpin. The outside of the bearing is completely covered in polished aluminum from the con rod - looks like chrome
I think I now understand what happened:
The big end bearing partially seized onto the crank pin; this has caused the bush to rotate in the conrod, having a polishing effect on the bearing & also blocking the lubricating holes. Then the bush has seized, breaking the bush & bottom of the con rod.
As I explained before, the engine was only being test run on very low throttle openings, following the instructions,etc & the throttle never opened more than 1/4. I also oiled the camshaft, rockers, etc before running the engine - so couldn't have done much more. Like I said - I've just been unlucky this time - & just to add to it YS are on holiday this week!
Next time! - I shall remove the backplate & well lubricate the big end, as well as oiling the cam, rockers etc. Once bitten, twice shy - as the saying goes.

Stuart