we've used Purple Parking for the last 3 trips and never had a problem. We booked it through Quidco almost a year ago for August.
I have to work the day before we travel and won't finish until 10pm as otherwise we would have been tempted to stay in a hotel the night before, you can get some good deals which include the cost of parking and you get the added benefit of twilight check in making the morning of your journey so much easier.