[SOLVED] Diagnose my starter problem please

[keratos]

How did you determine that the starter relay was fine? If it was by using the continuity range on a multimeter that could give a misleading result. The meter only passes about ten milliamps to measure the contact, the starter is trying to take several thousand times as much current, a few tenths of an ohm can be enough to stop it yet a meter will see good continuity.
Try measuring the voltage between the output of the starter relay and the battery negative.
If you get 12v (nominal) and the motor doesn't spin then you've an (intermittent) connection between the relay and starter or between starter and earth. If you get a lower voltage and the starter doesn't spin, it's probably contamination of the relay contacts.

I did both. Continuity and volts. All OK. The meter is a ut71 , £150.
You forgot to mention the starter cut off relay. I tested this too but it's a complicated box so often the best way is to swap it out and see. I may do this. It's definitive not the starter relay or the motor or the main switch or the run switch or the neutral switch. I can't think what else it might be