Yes I thought of that. The pump is OEM Bosch from FCP and came with a new check valve attached. I suppose (given current sourcing and qa) that it could have been bad out of the box.
I didn't favor this since I thought that if the check valve was bad two things would happen.
1. It would be a hard start (not in this case) since the line would not be pressurized
2. The problem would not take 10-15 hours to occur.

I imagine that if there was a minimal failure in the valve that it might take a long time for the pressure in the line to fall but that still does not explain the normal immediate start.

I will replace the valve and let the board know if that solves the problem.

Thanks
Howard