I'm thinking I should ignore it and see what happens. After 40 odd years with no fines whatsoever I'd hate this to blemish my record although it was a bit naive of me to think I could park for free near a major sporting event.
As everyone's said, this isn't a fine. Here's chapter and verse about parking on private land:
http://www.telegraph.co.uk/finance/perso...park-fines.html 
