G4S has broken their contract so they have no right to claim the agreed payment. They know this and are trying to re-negotiate the contract after the event. I suspect that their current claim for the £235m is really an opening position and they will be ready to lower it during the negotiation process. Nobody starts negotiating from their finishing position, it is a bluff.
Boris gets them a cheer so now they think its all forgotten, utter pricks, but yes it is probably their way of opening negotiations.