How about something along the following lines?
I am a loyal Sainsury's customer and the holder of a disabled motorist's blue badge. When I shop at Sainsbury's I endeavour to display the blue badge in whichever car I travel in. However, following a visit on [date] I received a PCN from your parking contractor for my car being parking in a disabled space without displaying my blue badge. It seems, therefore, that I might have forgotten to display the blue badge. I have attached a copy of the blue badge so you can see that my car was entitled to park in a disabled space and that no disabled person was wrongly deprived of use of the space. I shall be grateful if you will instruct your parking contractor to cancel the PCN. The details are as follows:
Parking Contactor: Euro Car Parks
PCN number:
Vehicle number:
Best regards,
NAME
You haven't told us whether you actually bought anything in the store - but if you did, and have the receipt or other proof of purchase such as an entry on a bank statement, you could throw that in the mix.
