In the next five minutes
Don't start the engine. Don't turn the ignition on — on many cars that primes the fuel pump. On a keyless car, move the key fob well away.
Put the car in neutral, release the handbrake and, with help from staff or other drivers, push it clear of the pump.
Tell the cashier. Then call a mobile fuel drain service and tell them exactly how much went in and whether the engine has run.
Why petrol damages a diesel
Diesel fuel lubricates the high-pressure pump in a modern common-rail engine. Petrol is a solvent and strips that lubrication. Once the engine runs, parts can wear metal on metal and send fine particles through the injectors.
That's why the fix is cheap when the engine hasn't run, and can be very expensive when it has.
What if I've already driven off?
Pull over as soon as it's safe, switch off and don't restart. Every mile matters. A full flush of tank, lines and filter often prevents lasting harm.
How much does it cost?
A mobile drain where the engine hasn't been started is typically a fixed price of a couple of hundred pounds, including fresh fuel and disposal. Pump and injector repairs after driving on can run into thousands.
How to stop it happening again
Keep the car's fuel type on your key ring, choose a forecourt with clearly marked pumps, and consider a misfuel-prevention filler cap or insert — they're inexpensive and stop a petrol nozzle entering a diesel filler.