After about 10,000 miles of use I decided to change the engine. The one in the car was running well and I wanted to keep it as a ready spare. Dianne was lucky enough to get a new (Phoenix) crankshaft for her birthday so Ian Bancroft had built a new engine for us. After changing the engine I had some small jobs to do on the old one. When looking at the bottom end with the sump off it became clear that all was not well. Pieces of the big ends were found on the sump gauze filter. In the end I found that three of the four big ends were breaking up. The block was cracked in two places. The crankshaft was cracked and the timing gears were damaged by bits of the big ends getting caught up in the timing gears.
Having thought about this damage, I have found a possible cause. Nearing the end of a long run the distributor weights fell apart. The engine did run rough, to say the least, the shock on the crankshaft could have given rise to this damage.
This crack in the block was repaired by stitching.
Cranks can go at any time but rough running would not help.
I assume bits of white metal bearing did the damage to these timing gears.