Write to whoever issued it and contest it.
I recently got a fine for £60 for "not displaying my parking ticket" however I had got a ticket -it was on my dash board (not a sticky backed ticket) and had blown over when I shut the car door.
I wrote off to the council (address on the fine paperwork) with a photocopy of my ticket and they have said I don't have to pay but gave me a verbal "slapped wrist"