if its not from your local police, or the local council. rip it up. bin it. be happy.
do NOT open into any contact with the company, and eventually they will stop chasing for the money. I got one in last december for parking in an unmarked disabled space (they even sent me a picture of me on the unmarked space), totally ignored it and got a couple of scary looking letters afterwards threatening court proceedings (the "fine" also increased from £60 to £200), then in about end February/march i got a letter in offring me to pay a reduced "fine" of £120, ignored that too and heard nothign since.
They wont take you to court over 50 or 100 quid, they will just chase the next person who will pay it.
the guide from moneysupermarket linked above is good. its all about trying to charge you for a contract that, as long as you refrain from contacting them, you have never entered into. that was my understanding of it anyway.