I know that the Sunday Trading Act restricts shops with a retail area of more than 280 M² to opening for no more than six hours on Sundays, between 10:00-18:00.
All the major supermarkets are represented in my city (with many operating from more than one site), yet all of them open 10:00-16:00.
Surely if one of them opened 12:00-18:00 they would have a de-facto monopoly on reasonably priced supermarket shopping for a 2 hour period between 16:0-18:00, why don't any of them do this?