Right, I have done a little looking into this, and thanks for the replies, very helpful.
France was an early adopter of the chip and pin, as such, it has a slightly different system/method/chip/something which means that some pre-paid cards do not work on the autoroutes and petrol stations. This was the main reason for having a card, not to faff about with cash at pressure points.
There is a huge difference between cards, there are costs, some of them are hidden. So, to make the drive down to the south of France that little bit easier I have bought a
Liber-T Tag.
Okay, we will still use cash at the petrol stations, they are not so bad, but the toll routes are tricky when you are on the wrong side of the car!
So cash remains king with credit/debit card backup.