Do Fuel cards on monthly dd save you money?

Hi,not sure if in the correct forum.
I have recently come across the Topax Fuel card which is a card you get from them and you pay for petrol once a month by direct debit
.Has anybody used this card?
Does it lead to savings on fuel costs?
My thinking also was that you would not be withdrawing extra money from atm,ie i usually take out more than petrol costs and spend on other crappy items in the petrol station and this might reduce this?

I think you are restricted to certain stations, which reduces your ability to shop around for the cheapest fuel.

I always pay by credit or laser card, which would solve the problem of carrying extra cash.