Hi Terry,
Just visited the Auchan on Thursday, returning from work in Rotterdam.
Price for diesel is around Euro 1,15 and Petrol around Euro 1,45 in Northern France.
The Auchan payment kiosk is manned from 08:30, and UK credit cards will NOT work in the 24 hour automated pumps. Mind you the saving at the Auchan is not so great compared to a few years back - only a few cents a litre.
So as others have recommended the 24 hour stations near Calais could be the answer for you.
Robert