Instructions:I found this site that uses less well known airport parking operators
Saved £10 on a 4 day stay to Liverpool airport
Receive a 15% Discount Off Airport Parking at London Heathrow, London Stansted, Liverpool, Edinburgh and Exeter
12% discount off Airport Parking London Gatwick, Birmingham, International and Leeds Bradford
10% Discount Off Airport Parking at Manchester, Bristol, Doncaster Robin Hood, Aberdeen and Belfast.