As we moved off again she decided to have closer look at the receipt, it having occurred to her that £21.40 was a bit much for a few groceries. It seemed the three biscuits she had bought for us has somehow been charged as 32 biscuits and while they were doing a wonderful deal on these biscuits (which they insist for some reason on calling 'cookies') whereby the knock 9p off each one bought as a set of three, 32 of of them was still a bit over the top.
Luckily we hadn't quite reached Lynwood Drive yet so I did a quick drive round the block and she went back in to sort out the issue.
To their credit the staff there sorted it out immediately, organised a refund which was of the face value of the biscuits before the discounts were applied so she ended up getting a couple of pounds extra back too (Hooray).
No discredit to the person who made the mistake to start with; they probably just hit the 2 by mistake after pressing the 3 and neither they nor Mrs WP spotted it at the time - an easy mistake to make. However it was good to know the staff were ready to correct the problem without question afterwards. And the biscuits weren't too bad either...