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Thanks to all for your answers, and, yes, I know that you should always wear a seatbelt. lol. I took the dressing down badly because it was handed out in such a condescending manner.
Pippa68, I have just checked with the council and have been assured that Sainsbury's is NOT classed as a public car park, it is owned by the company and, as such, is private land. If it were a public car park, it would be controlled by the local council.
I also phoned the police station. The desk sergeant was surprised I had been pulled on it, asked if I had been seen by the officer coming into the car park from the road not wearing a seatbelt and then told me that it was very unlikely that any action would be taken, even if I was racing around the car park and doing handbrake turns and wheelspins, unless I actually (god forbid) hit someone, damaged property or the store complained. Seems dangerous driving isn't as important as being on the public highway putting your own neck at risk.