Further Investigation took place tonight of my recently rebuilt, but now removed engine. I had some assistance in this matter and I was lucky enough to rely on their expertise and guidance.
The first problem we found was that only two out of the six caps on the big ends were even tight. On numbers one and two the caps were so loose that they were able to move side to side and connect with the crank when spinning round. When we did this and it made a 'clatter' type sound I recognised this as the noise I had been hearing since the Scottish rally.
Removal of the big end bearings also showed excessive wear on some, but not all of them. Although the bearings with excessive wear also didn't seem to fit the caps properly, so this needs to be investigated too.
Luckily, the crankshaft doesn't seem to have been damaged, so a replacement set of 'decent' big end bearings should be all that's required. (Hopefully!) We also found some slight pitting on a couple of cam followers, so a new set of these will be ordered too. To be continued..............