I would have tried this service as it seems like a good idea, however as I'm leaving and returning to different terminals they say you need to ring them instead of booking online.
It's 10 p a minute and they don't bother answering the phone. They assure you they will and they are busier than usual but after spending several pounds over a couple of days waiting I still hadn't been put through. Who knows how long it would take if there was a problem and you tried to ring to solve it.
Waste of time using this service.