Stuarty - ignore IGNORE ignore!
Do not contact or acknowledge them. John's advice above is sound - plenty on pepipoo as well
I have a lovely wee collection of letters as a result of parking in Gallagher retail park in Dundee in October 2013 (which is free for the first two hours but you must display a ticket - Gary didn't realise this)
The letters begin with the usual 'reduced payment offer' and progress their way through several different companies (all one in the same) who threaten court action, CCJ's (they don't even bother to change the wording for Scotland), and eventually tail off.
The only fines you must respond to are from Police and Council.
http://www.thecourier.co.uk/news/scotland/drivers-urged-not-to-pay-private-parking-fines-1.251312