Have you thought about driving to the airport yourself, dropping everyone and luggage at the terminal and one person going off to the long term car park and getting the courtesy bus back to the terminal. ! The prices aren't that steep.! Heres a link:
http://www.holidayextras.co.uk/partners ... gent=WR765
The other alternative could be as someone else mentioned Addison Lee, they are very good, a bit higher priced than some local mini-cabs but great service, they also do a pick up service for when you arrive back !