i've been doing the same
I've been doing the same for the passed few CPL sittings and have been using Hotwire.com and requesting hotels in the Gatwick area and have had some crazy results. I got a 5star hotel there for 22quid per night the last time.(The breakfast was more expensive than the room per night !!)
I've done the same for this next set of CPL exams in July and got the holiday inn Gatwick for 30 pounds per night.
Well worth a go.