[jambo]

Breaking out the trusty Haynes manual.

Given *everything* dies and the main live & earth terminals are solid (you've checked both ends of these cables not just the battery end).

The main ignition live runs as a white line from the Starter relay, to the ignition switch, then as a brown wire from there back to the fuse block.

Check the connections are good at the starter relay, as well as any connector blocks along the way. Other wires would lead to failures of ignition, lights, or other systems but I don't think much outside of this loop would affect everything simultaneously as you describe. Note the wires, connectors and components on that route can all affect this, I would pay particular attention to the main starter relay and plug for the ignition switch into the loom.

Jambo