As the mercury hit the 90s and the UK sweltered under record breaking temperatures, we put the major online retailer's operations to the test and turned up some surprisingly refreshing results.

All five of the retailers under our spotlight managed to keep their cool and provide our internet ­guineapigs with a virtually stress-free ordering and delivery experience for the 33 goods in The Grocer 33 list on the previous page.

As the popularity of online shopping reaches new heights, our shoppers are having fewer and fewer issues with the technical side of the ordering process.

Our Asda shopper was tripped up by a minor glitch when she reached the payment stage. However, a quick call to the retailer's helpline soon ironed it out.

Our regular Ocado shopper also ran into a spot of bother when she tried to use a £15-off voucher she had received after a previous order. The system did not recognise her code and our shopper was ­unable to find details of the terms of the offer on Ocado's web site. She added that she found the search button function a bit clunky.

However, when it came to choosing a delivery slot, four out of five retailers were able to offer a next day service. In addition, all deliveries were made within their designated slots, although Asda's arrived with minutes to spare.

The issue of substitutions is one that has got our mystery web shoppers a little hot under the collar in the past, but again all five retailers excelled this time. While Ocado was the only one to provide all items ordered without a single substitution, the remaining four all offered perfectly acceptable alternatives for any products that were unavailable.

Asda replaced a Quiche Lorraine with a tomato and basil variant and our shopper said the delivery driver checked she was happy with the switch before he left. He also made her aware of which bags contained frozen products.

Tesco provided own-label Carbery mature Cheddar in place of the Cathedral City our shopper had ordered, while our Waitrose shopper received reduced fat ­houmous rather than the standard variety ordered.

However, if there were an award for the most improved service, it would have to go to Sainsbury's. While it still substituted one item and was unable to deliver another, this was much better than our last report, when it substituted six items and failed to deliver one. The delivery arrived bang in the middle of its designated slot and the driver took the time to explain the substitution, and also helped to carry the bags into the kitchen.