Tesco has won our Top Store award this week. Its victory follows an upset by Somerfield last week, breaking several weeks of domination by Bradford-based Morrisons.

Our mystery shopper at the Tesco superstore on Newland Street in High Wycombe said its attentive staff, convenient location and full offer of both grocery and non-food products made a strong case for the traditional full service high street supermarket. Store manager David Sherlock's focus on availability and checkout speed paid off as she whizzed through the store quickly and was able to pay for her goods immediately.

Previous winner Somerfield put in another strong performance this week, confirming that last week's victory was no fluke. Our mystery shopper at the Somerfield store at Edgeley, Stockport praised the staff who maintained levels of service excellence that our shopper didn't expect to find at a basic shop in a working class neighbourhood. However, the store slipped up on availability.

On the other hand, our survey uncovered negative surprises this week, such as an uncharacteristically poor showing by perennial service award contender Morrisons. Our mystery shopper said Morrisons' store in Yate, Bristol, was dirty, with opened boxes of food visible and empty banana skins left on shelves. Her ordeal was punctuated by long checkout queues and an unfriendly checkout assistant.