At the risk of bending your credit card out of all recognition, look at the commercial area just inland from Boulogne on the main road to St. Omer
The only downside, apart from the stress fracture of the credit card - is that the pumps are, as usual, fairly caravan unfriendly, but there is everything else there you could need - auchun, and enormous bricolage etc. etc.
It's only a mile or so inland from the Calais - Caen road and hectares of parking - you could do a lunchtime stop there and no-one would notice.