[scottb]

On this side of the pond, Techron is a product made by Chevron. It cleans out the fuel system, and does a great job on P-cars. I don't know if it's sold in the UK, but I'm sure there is something else there if it isn't.

If you've got a bad load of petrol, you probably have only two options. The first is to drain/siphon what you've got in the tank, and get a fresh tank of fuel. The second is to keep driving it and periodically add fresh fuel as the tank empties (every five gallons or so). If the tank is relatively low now, take it to a filling station you trust, fill it up with the highest octane fuel the car will tolerate and see if that helps.

Good luck, and let us know what happens.