Instructions:Received by email:
We are offering you an amazing 20% off (on selected products)
Enter code E79W
Offer valid until 27 Sep 2010
No idea what they mean by "selected products" but the discount seems to be coming off for most airports. The expiry date is for making the booking, not the actual parking period. It seems to offer a voucher for £5 off Tesco grocery shopping when you book certain services, like park and ride at Heathrow and APH at Birmingham- added bonus!
Don't bother with cashback sites though - they will just reject the transaction.