I recently left the engine running when I parked my van in a private car park at Tesco to go to the cashpoint. A police officer asked if it was my vehicle. I said yes and said I was going to the cashpoint. He said, "Do what you've got to do then come and have a chat". When I spoke to him, he issued me a ticket for leaving my van running. I wrote to the ticket office and pointed out that he had encouraged me to walk away from my vehicle with the engine running, rather than ask me to turn my engine off. I also pointed out that the ticket is not filled in correctly, as it doesn't mention the car park I was parked in, just the main road junction I was near. Rule 123 of Highway Code says the vehicle must be on the public road for this offence. I have photos of the signs saying privately owned car park where I was parked. I pointed all this out in my letter but they have still issued me with a conditional offer fixed penalty of £50. I believe I have strong grounds to challenge this. Should I challenge it or just suck it up and pay the fine?