If you are trying to get back less than £5,000 from G4S, then just take them to the small claims court.
Although I've never done it myself I gather it's fairly painless. You don't need a solicitor (so no real cost to yourself) you just fill in the cliam paperwork and represent yourself in court.
Venues and forms
Needless to say, as soon as G4S are informed of your case against them I would expect them to settle out of court. If they do go to court you could always infrom your local paper who might send along a reporter......