If we assume that the number and location of major supermarkets is planned in relation to local population demand, as is the undoubtedly the case both in Scotland and in England & Wales, then bearing in mind that there have been no Sunday trading restrictions for years here it is the case that where I live there are now more larger supermarkets than 5 years ago - a large Waitrose has just opened that is with a mile or so of large Asda and Tesco supermarkets and they all open as normal on Sundays.
This seems to go against the idea that extended Sunday opening somehow isn't profitable since, if so, and since we are told premium pay rates apply in Scotland, then surely by now we'd have seen the major retailers operate reduced Sunday hours in their large supermarkets here in Scotland for commercial reasons - and they haven't.
For some reason extended Sunday opening oddly seems controversial in England & Wales, hence the previous threads on this, but normal here - so I'm suspicious that this is more about politics (both Usdaw and SNP) than it is about commercial concerns: I suspect this cost excuse is a red-herring.