Afaik SCs located in paid-for car parks are a very tiny minority of locations you can count on one hand. Others are free or you just have to validate your reg at reception.
On the contrary, Euro Car Parks, which controls the majority of Sainsbury's locations have some tricky T&Cs displayed, typically 1 to 3 hours free parking for 'customers', but only 10 mins outside of store opening hours. So unless they change this, you could receive a £100 PCN for charging after hours, or if they are sly enough, the same PCN if you haven't bought anything in the store while charging...