Just found this on another forum!
Apparently the store only realised the mistake, after police asked to see the manager, as the cars were queing on the road, round the corner, and could someone please come and “sort it out” 
Asda got loads of publicity from this, it was all over the radio and TV on Friday and bearing in mind Friday’s date I have my doubts about how much of a ‘mistake’ this was. Anyone from Asda who was interviewed managed to get in how their fuel is usually cheaper than elsewhere, although not usually this cheap. Clever bit of marketing if you ask me.