Instructions:Been looking for a park around heathrow airport, found this offer which is for NHS employees but the link should work for everybody.
I tried before booking by their normal website & after using this link (http://www.purplebusinessparking.com/) and the discount was true. However the discount might vary per airport or duration of booking.
NB: there should be a 15% topcashback, didn't work well as there is no place to put the code (brnh2) on their website so I had to change the link (which I would assume as a result invalidated the cashback)
Another code tried was brnhs which also worked but the brnh2 gave a better rate. Expires 31/03/100