Halfords do standard retail park opening hours.

9-8 Mon - Fri
9-6 Sat
10-4/11-5 Sun

I would imagine you would do a 9 hour shift 5 days a week or 6 on a Sunday.
45 - 48 hours Max

I imagine the rota would be split 9-6 (30min lunch 2x15min breaks) and 11-8 with the same breaks.

Saturdays 9-6 9 Hours worked
Sunday 6 hours worked

Depending on the size and how busy the store is there may also be a 10-7 shift to ensure the busy rush hour 5-7pm slot had maximum staffing levels.

I dont see how there could be a possible problem

At the store managers discrestion what hours you worked could be spread over days that you didn't particularly like or make it so that you didn't have 2 days off together.

Thats the shakes of working retail, a good manager may be able to resolve this issue with the staff and negotiate however ultimatly staff shall work to the demands of the business.

Its possible that there are other constraints on head count and staffing allowance... or he could just fancy a change.

That is pretty much retail management of staff - I used to run a team of 12 plus 1 repair 1 admin and 1 installer at a retail park Carphone Warehouse.