YABU to call it a "ticket" or a "fine". It is a speculative invoice. That's legalese for a try-on. They are made to look like official parking tickets because it helps to scare people into thinking they're real. They're actually about as legitimate as a nine-pound note.
These speculative invoices are without foundation because there is no reasonable preestimate of loss. £5 would be a reasonable penalty charge. £85 is out of all proportion. There is no sensible argument that shows that Parking Scum has lost £85 or whatever because someone didn't get a blue badge stamped in a car park that presumably had more space in it anyway. Ludicrous.
Ignore it, and ignore any bullying letters you get afterwards. Do not respond in any way as to do so is to acknowledge its validity and confirm your whereabouts. If you write back, appeal, or protest, what happens is that they or their "independent" appeals panel decides that you are due to pay up notwithstanding your argument, and that since you used the appeals or complaints process you must have agreed to be bound by it, so its findings stick. Instead, let any matters be adjudicated by the competent authority in your jurisdiction, i.e. the Small Claims Court.
In the few cases where they are actually stupid enough to take someone to court, they either fail to turn up, since their bluff has been called; or else they get bawled out by the magistrate or sheriff and are lucky if they don't get cited for contempt. Their use of the courts to pursue debts that they know are not due has on some occasions been held to be a contempt. The only defence you need is that £85 (or whatever) is not a reasonable preestimate of loss. Parking Spy's own claims about people they have successfully sued are lies.
If you are in Scotland know also that they cannot require you to disclose who was driving. They now can in England and Wales since the "Protection of Freedoms" (sic) Act became law, but this does not apply in Scotland. Even south of the Border it makes no difference to the preestimate of loss point.
The people that send these speculative invoices out are the most odious and disgusting parasites and revolting scum. They make their profit by persecuting and frightening vulnerable people and threatening them with baseless threats of legal action.
If you must write to someone, write to your MP asking him to prevail upon Parliament to pass a law requiring the likes of Parking Lie to be prosecuted for fraud, extorting money with menaces, holding the courts and Parliament in contempt, and generally being the vile bastards that they are.