I missed the bit about only the presence being testable, which does certainly suggest the use of a nyloc or split pin is mandatory - But that doesn't explain the cars I've owned (and possibly still do, I've forgotten exactly how the ball joints are fastened on my current lot) that have used plain nuts with spring washers. But mandatory or not, a five minute trip to the motor factors to buy a nyloc of the correct size should still have been enough for the MOT man, and stopped this thread expanding!
