One of the last tricks my MGBV8 played was this, and I spent a lot of money being too clever for my own good. New starter, new batteries, checked all the connections. This was over a few weeks and nothing made any difference. It turned out to be a hairline crack in the positive battery terminal connector. In a final act of desperation I put a spanner on the clamp for one last pull. When I did this, it disintegrated into two parts. I felt relief and stupidity in equal measures, and had spent over £200 to fix a £3 clamp.........
Moral? - don't be too clever for your own good. (Until next time).