You have several options depending on your vehicle, how much petrol to diesel ratio is in the fuel tanks and how brave you are feeling:
First off, if your diesel car is new (after '05) then the engine is most likely of the 'common rail' type.. DO NOT start the engine, call a mechanic and have the car towed to a garage where you can have the fuel tank flushed for a not reasonable fee of anything from £100 upwards.
I'm not sure if I have a 'common rail' diesel engine. Ok, common rail diesel engines are more popular in modern diesel cars, newer cars sound more like petrol cars, explaining the rise in mis-fuelling of diesel cars. Normal audial clues are not there with the smoother running modern cars. If your car is quite new then DO NOT start the engine. Some new cars even come with a diesel pre ignition system, this lets the car pump and pre-heat the fuel as you open the door.
To be on the safe side, DO NOT unlock the doors or de-activate the alarm until the mechanic arrives, make him aware that you know of this, even if they do not. I hope you haven't left your phone in the car. If your safety is at risk though, just get into the car and lock yourself in, the damage will be minimal, if any, compared to if you started and ran the vehicle.
What happens to the diesel engine when i put petrol into it ? Petrol breaks down the oily lubricant in diesel that makes it so hard to get off your hands and the soles of your shoes. This oily substance lubricates the engine while running, without it the engine will start making a nasty noise and will stop as the piston seizes against the engine block due to over heating, more than likely costing you a new engine.
That's really being on the safe side isn't it, I have put a couple of pounds worth of petrol in my diesel car and realised my mistake, will it be ok ? If you fill up with diesel you should be ok with an older model, roughly early to mid 00's and older. If you are not sure DO NOT risk it, the more you run a common rail engine with petrol in it, the more it will cost you. Colder climates regularly put petrol into diesel to help stop it from freezing but using older vehicles. A good clue is: does your diesel vehicle make the classic clunkity clunk noise of a diesel engine, and is it a bit beaten up ?
If the answer is yes then you are probably safe to drive it with up to a third petrol (maximum) and no thrashing it, if the engine starts to sound progressively noisy then it would be a good idea to pull over and call road side assistance. If not, keep topping up with more diesel as often as possible to dilute the petrol. If you can afford to, drain the tank at the first opportunity and re-fill with the correct fuel. You could allways use the drained fuel for starting bonfires..
Am i covered by insurance for mis-fuelling my car ? In most cases the answer is no, as with most things the insurance companies will do all they can to get out of paying out. I would assume that they see you putting the wrong fuel in your car as your own fault, therefore your problem.