Another visit to the store may be in order - whilst the manager seems to have been fairly helpful, one likely reason UKPC didn't listen to them may be because Homebase are not the ones who hired them. As this appears to be a retail park, it's likely that the owners of the retail park, or their managing agents, hired UKPC. See if the store is able to put you in touch with them, or alternatively try to find their details and make contact.
On the face of it, it would seem that the driver has breached the terms of the contract, so as far as UKPC are concerned (and probably POPLA), the charge has been 'correctly issued'. Unless you can get the retail park to intervene this could be a hard one to beat.