Ideally you need a better pic - their allegation is 'left site' but if that is not on the signs then there's nothing to enforce against. It says 'Customers only' which you were, but can't see if it also says need to stay on site. Worth a try with B&Q to see if they will quash it.
Note, you need to avoid 'I parked' - refer only to the driver in the 3rd person. There are likely some deficiencies with their PCN to allow transfer of liability to keeper.
