Instructions:Title is self explanatory.
Managed to get myself a Heathrow parking in Easters between 20 March and 24 March for £17.50 (£3.50/day) !!!!
(TIPO code works as £29, but if you extend your stay a bit extra, TIPO works as £19!!! Strange ...)
I got the codes from here: http://www.myvouchercodes.co.uk/discounts/purpleparking.com