Ignoring is certainly not the recommended route for all private parking firms any more - that advice as a "blanket response" was safe up until around 18 months ago.
Then private firms did start taking people to court and far more worryingly actually winning.
If you want actual advice from people who do know what they are talking about, pop along to
www.pepipoo.com and ask there - advice would be to scan in both sides of the notice with your post (as the people there will ask you for it). You'll actually get advice there which is tried and tested (and yes, may well be simply ignore).
Or just stick to the blanket "ignore/destroy/send them an envelope of
**** back through the post" usually trotted out here - but I personally wouldn't recommend that option.