The vagueness of the BBC page is what prompted me for help and clarification. The later referenced page (above) is much more helpful. This evening, we went shopping at Morrison's, and they had a petrol discount coupon offer. How convenient! So I filled up the car, then filled up the red metal can, and presented my fifteen-minute-old coupon for 12p off per litre. It may have helped that the car's location completely obscured the can from the attendants, but it worked fine.