Petrol prices are always something of a talking point in the UK. The price we have to pay at the pump on petrol is more than most other countries (over double the price it costs in America), and with prices never seeming to go down much it can be quite costly when putting petrol in your car.
The money saving idea for this is one that is actually promoted by many petrol stations anyway in their attempt to make you "loyal" to that specific petrol station or the brand of petrol station e.g. Shell or BP.
What is on offer are cards which you can collect points when you spend, so for example, every pound you spend on petrol equals one point. These points could then be exchanged for either money off a petrol bill once you reach a certain number ora free gift. Many supermarkets now have their own petrol stations, and points may be exchanged for money off your shopping bill, this keeps even more money within the company and is one way that they retain and keep business to their stores.
It can also sometimes work the other way round too, such as when you do your shopping you may be able to get petrol from the supermarkets own petrol station cheaper, such as 5p off per litre, which if you fill right up can work out quite a good easy saving.