I pay for all of my fuel with the Santander 123 Credit Card which give me 3% cash back on a maximum combined monthly spend of £100 (So £3 max per month). I also try and fill up a shell fuel stations and use their points card which gives me some money off vouchers every 3 months. Overall though, the savings don't really add up to much.
That doesn't sound right to me. I also use a Santander card, and only use it for fuel, and anywhere that doesn't accept American Express, so not much in the way of "extra" cash back over the petrol. I'm due about £150 back this year with it. I thought it was £250 per month on "transport", it was when I took it out.
I also buy my fuel from Morrisons usually. Along with my shopping. Their miles and more give you a £5 back after 5,000 points collected. Points are mostly gained from fuel purchases here too.
I also buy gift cards from them. For every £10 on gift cards, you get 1p off per litre. So whenever I buy from Amazon / Argos / wherever they sell gift cards from, I buy them at Morrisons. Often get 10-20p off per litre with them. Great with my Range Rovers 100L tank. But usually manage 63-65L in both the 5 series and Alhambra too.
So I usually get 3% off card, probably average another 3-5% off gift cards, and whatever off the miles and more on top (I only spend the £5 vouchers on shopping though to maximise the cash back on fuel).
I do around 25k per year, so it adds up.

