Right, thought about it some more and driven the car today.

If it was the driveshaft or propshafts, I expect the frequency of the rattle would decrease as the car slows down, and yet, it doesn't.
Sometimes it's a really fast rattle, it slows down, stops, then starts again, totally out of control.

It actually sounds like a metal plate rattling about

I am thinking it may be the gearbox now... shifting from 3rd to 4th and back to 3rd repeatedly at a standstill, engine off, the noise is sort of there, clink, clink with every shift

Could something have rattled loose in the gearbox causing the noise ?