At risk of sounding like a pedant, the overstay was 11 minutes, not 1 minute. The grace period is not part of the "allowed time" as such, but rather exactly as it says, a grace period to allow for things like leaving the car park etc.
The argument from ECP (which to play devil's advocate isn't entirely without merit) will be that enforcement has to start somewhere. Arguing de minimis might be hard.
If it were me, for the initial appeal I'd avoid muddying the waters with PoFA or de minimis be focusing on the fact that the reason the vehicle overstayed was due to a breakdown. The driver was prevented from complying with the terms due to circumstances outside of both parties control. This would seem to be a clear case of frustration of contract.
If you haven't already, also approach the supermarket to see if they'll intervene.
