Have done that twice - second time I was at the pumps when I realised my mistake. Rang garage and advice was not to move / drive it. Diluting it won't work - or so I was told. Better not to drive it and it might be only a case of getting it flushed out. Otherwise might have to replace parts.
If the car hasn't been started then a flush of the contaminated fuel will sort it - the guy who picks you up from the forecourt will usually do this. If it's been started and run for any length of time then you may need to look at replacing injectors at least.
On TV3-IrelandAM this morning they had the AA on showing a mobile fuel flushing vehicle where they will come to you and flush out your engine (for a fee - wasn't cheap).
Just an update, spoke to my local mechanic (not a dealership but know him for 10 years) and after he told me I was a fool for doing this he advised that no need to flush just drive and top up constantly. I have driven over a thousand miles since, no problems yet.
Maybe it depends on how much you put in and / or how much you can dilute the petrol. I put in more that 1/2 a tank so it would have been hard to dilute out the petrol.
As I said I have done this twice and its the first time I heard that topping up with diesel will resolve the issue.
I have also put petrol into diesel car before but topped up remainder of tank with diesel. So as long as its diluted down considerably it'll be ok. if it was diesel into a petrol car then its dangerous.
Just an update, spoke to my local mechanic (not a dealership but know him for 10 years) and after he told me I was a fool for doing this he advised that no need to flush just drive and top up constantly. I have driven over a thousand miles since, no problems yet.