joa44a, the situation you describe is so common that it is an annoying problem for the police in many ways. Forecourt crime is massive, however, the majority of reported Making off-Without Payment (MOWP) offences are infact usually errors made by cashiers working for understaffed garages with shoddy security policies (if any at all) and hence a civil matter.
Such situations are then reported to the police and made out to sound as if it was a deliberate and intented act by the customer. The police will obviously enquire what exactly happened and it is common for the complainant to state they asked the customer whether they had any fuel and was told no. Garages know that if they can persuade the police that it was a crime then they have to investigate it and will recover the money at no cost to them. (In summary, garages can actually request registered keeper details etc from DVLA, but this means they will have to fill out application forms and pay a small fee).
If such a report as you describe gets recorded as a crime with the police then they will do all relevant intelliegent checks and if not considered a criminal act then they send out a letter to the registered keeper of the identified vehicle suggesting you go and pay if it was an error etc. Once paid, the garage is happy that they have recovered their money and do not wish to take any further action. The police then write the report off as a civil matter which has been resolved (still takes a fair amount of paperwork and time).
I suggest if you wish an explanation and apology then write to WM.
(PS I had to rush writing this answer, just wished to highlight to you that this is a very common issue)