Last Saturday I travelled from Chelmsford to Norwich & was surprised to find a much greater variation in petrol prices from service stations on the A12 than is found in Chelmsford & it's surrounding areas.
Two places on the northbound carriageway north of Colchester were selling at £1.43/44 whilst by contrast, on the return journey, there was a single Shell outlet at £1.63 !
Apps. are not really any use to me as all unleaded prices around here are much the same, bar a Shell outlet close to me which is always at least 4p higher than the rest.
The only saving grace for me is that I can buy E5 protection grade for my (occasional use) classic at Tesco for little more than E10 (used in our daily driver) from the Esso/BP outlets.