Parking at Heathrow

[suicidal_monkey]

I need to park someone's car at Heathrow so it's ready for them to pick up the next day. We each have a key but if there's a parking ticket I'll need to be able to leave that in the car. Anyone got an opinion on where is the best, and not too horribly extortionate, place to leave a car for between 24 and 48 hours near Heathrow (t5)? :-)