In reply to David Barratt:
VCS can't issue a debt - they're just a private company. But a county court can.
The relevant law is the Protection of Freedoms Act 2012. Under this, VCS can get your details from DVLA and, theoretically, issue a county court claim. If you don't respond to this, you will get a legally enforceable debt registered against you. I don't know if VCS are one of the companies which tend to do this or not - I've a feeling they might be one of the ones that just issue several scary letters and then give up - but even if they aren't litigious at the moment they could re-open the case any time in the next six years. The company that used to control all the Co-op's car parks has just issued a huge number of years-old claims in the county court system, and a lot of people have ignored these claims and then had real CCJs issued.
Seriously, go to PePiPoo and get advice on responding to the claim. VCS will initially reject it (because they reject every appeal regardless), then you'll take it to the independant appeal process ('PoPLA'), then you'll win - and the claim is gone forever. But this route is time-limited so don't wait too long.