Has anyone fitted a check valve in the fuel line of a Thor V8.
I have a new valve and 8mm pipe but not sure the best place to fit it.
It drains back overnight and takes a few minutes to fill up again.
I have fitted a new pump in the tank but its not changed things.

Your pump is suspect. They have an anti-drain back valve in them. Clearly yours is stuck open or something leading to the issue you have.

Cheapo pumps often lack the NRV (check valve). Can cause quite a problem with diesel variant so we fit a check valve on the -return to tank line- in engine bay to prevent it running back, or spend £120 on a VDO pump that has proper OE spec. Not sure what pump is relevant to the petrols but the run back to tank will work the same.