Whatever the case, a retailers or manufacturer's T&C cannot override present consumer and unfair contract laws.But getting re-address can some times get very difficult, especially if the other person 'knows their way around' these things.
Because a suppliers vehicle breaks down (because its the oldest in the fleet) as nothing to do with you, and you should not lose on this, by paying extra for the service being provided.
I would suggest that you visit your local CAB, Legal Advice Centre or contact http://www.consumerdirect.gov.uk for further advice.
You could also check to see if the supplier belongs to a trade association. If they do, then there might be some help in resolving any issues there. But I have found on previous occasions, that some of these trade associations can be more for their members, than the general public in resolving issues.