How do you shop for petrol in the capital? Is it simply a case of finding the nearest and most convenient service station, or are you influenced by other factors, like a helpful and ever-smiling cashier? Maybe you are amassing an impressive collection of gift tokens in time for Christmas. Perhaps you appreciate the forecourt facilities like the car wash, or the shop selling essential barbecue supplies.

Just possibly though, price may be important. If you don't already look at the cost per litre, perhaps it is time you started to pay attention. London is a unique battleground for petrol retailers, with millions of motorists and thousands of outlets, and a few yards can save a small fortune.

Let's do the sums. A Ford Fiesta holds 42 litres and let's assume you brim the tank once a week. Find a forecourt that undercuts your current favourite by just 1p and over a year you will find an extra pounds 20 in your pocket. But in London you can save as much as 12p per litre.

So why the difference? Well, it is worth remembering that

75 per cent of the cost of petrol is accounted for by tax, half of the remainder is the cost of

production and distribution, which leaves 12.5 per cent for the companies to play with.

Robert Wine at BP said that it and the other companies 'are wholesalers, selling direct to the retailers, who then price according to the competition within their area. This must be true because Mobil refused point blank to talk to us, citing 'competitive reasons for its reluctance to discuss the

London region.

It all boils down to how many service stations there are within a locality, whether the site is on a main road, a dual

carriageway, or buried in the suburbs. At Shell, Andrew Vickers said London was not subject to a particular policy. 'Nationally we simply aim to be competitive.

An increasingly important factor and additional profit centre are the extra facilities offered by the service station. Jet's slogan is 'a shop on a forecourt' and Kim Creed stressed that 'we want to be seen as a convenience store with the Jiffy shops generating extra cus-tomers. As for petrol prices, we always aim to be 5p below the major brands.

In this sector the retailer that has made the biggest impact is Tesco. Its aggressive pricing has pleased shoppers and outraged competitors. Accusations that the absence of 'detergents that clean the engine accounted for the price discrepancy were soon rectified by Tesco. In London last month, Tesco took the petrol retailing war a step further by opening two 'Tesco Express independent filling stations. Located in Castelnau, Barnes, and London Road, Thornton Heath, they have combined cheap petrol with superstore prices in the forecourt shop. If you have ever felt ripped-off by a premium-price petrol shop, these new outlets are the answer. It is also reassuring to find out that the Tesco pricing policy is designed 'to be the cheapest, or one of the cheapest in the locality.

To find out how the price of unleaded petrol varied we drove along some of the capital's main routes, jotting down the highs and lows. Starting at the east London end of the North Circular, the dual carriageway approach known as the Southend Road, you could top up at the Texaco station for 50.9p a litre, but on the opposite side of the carriageway Shell was 47.9p. If you were prepared to make a diversion around a couple of roundabouts to save one penny, then on the Chigwell Road accessed by the same roundabout was a Q8 outlet asking 46.9p.

On the North Circular proper, a pattern emerged as the petrol got progressively cheaper. At Chigwell, Mobil pitched in at 51.9p, only to be bettered by BP at 49.4p a little further on. Jet proved its commitment to low prices at Palmers Green.

Its 48.9p set the standard for the subsequent Fina and BP stations to match, but by far the lowest was Tesco, near Brent Cross, at 47.9. Further west, the price settled at 49.9 at Mobil, Texaco and Shell when the North Circular became Gunnersbury Avenue.

Then there was a blip on the scale as a BP station not far from Chiswick roundabout mounted an impressive 46.5. Heading towards central London on the A4, the prices began to climb again: 52.9 at a BP on the

Talgarth Road, then 53.4p at an Elf in Hammersmith and the Euston Road. But the top price went to CYMA, a small independent company opposite the Shaw Theatre, at 57.9p.

On the way out of the West End a clutch of Texaco garages competed with each other at 49.9p, 50.9p and 48.9p, the former just yards apart in

Bethnal Green.

Beyond llford a clutch of petrol stations signalled a small skirmish: Esso 47.9p, Jet 46.9p, Fina 45.9p Esso 45.9p and Texaco 45.9p. So who exactly was responsible for this outbreak? Well, Tesco, located in Goodmayes, seemed to have a lot to do with it.

As a fully paid-up member of the service station spotting society I can confirm that it pays to shop around. It also helps if there are plenty of petrol stations to choose from on a busy main road. Best of all, make sure there's a Tesco nearby.

(Photograph omitted)