Hi Wayne,
I'm British and I live in France. What Val says is correct.
However the problem is not our British credit cards it is the French card readers. Some have the latest technology enabling them to read ALL cards, British included, but some do not. We do NOT use the 24-hour unattended petrol pumps unless we have our French credit cards with us.
What Val recommends is a good guide. Having Euros cash with you and asking a local to pay with his/her card is a good move if you can't find an alternative. This actually happened to me when a FRENCH man's French card was not accepted - he gave me euros and I used my French card.
However, if you're travelling on the autoroutes then you should have no problem having your UK cards accepted.
Otherwise do as we do, always top-up during normal working hours at a manned pump.
Dave