Marston Group appears to be a legitimate business:
http://www.marstongroup.co.uk/
They can only work from the information which has provided to them. So you need to challenge the source of that information, rather than Marston Group. If they've been notified of an unpaid fine it means that a court must have issued that fine. (i.e. Marston Group are not only assuming that you failed to respond to a demand relating to driver details, but also that you ignored a subsequent summons to appear before a court, and the notification of the fine issued by that court in your absence). You need to insist that Marston Group tell you the name of the court that fined you, and the date upon which that fine was issued.
When you know the name of the court you can get the contact details from this link:
http://www.hmcourts-s...v.uk/HMCSCourtFinder/
(NB: The HMCS web server is terrible. It frequently seems to stop working outside of office hours. If that link doesn't work, just google the name of the magistrates court for contact details).
The court officials should be able to provide you with information relating to how you can challenge the fine on the grounds that you never received a summons.
Chris