Large shops (more than 280 square metres of sales space) can trade for only six hours on a Sunday, and these 6 hours can be whenever they choose. Small shops can open for as long as they want.
Certain types of businesses, such as petrol stations, motorway services, farm shops, pharmacies, shops at ports and airports etc. can trade for as long as they want also.
Also, some large shops that are affected by Sunday Trading hours will open for longer than six hours... maybe 30 mins or an hour extra at the start of the day, when customers can browse and choose products but not actually buy them until trading hours.