So here's my solution:
1) The Government should cancel the contract with G4S immediately and refuse to pay them a single penny. Let them sue if they dare. They are a bunch of halfwits who cannot be trusted and will simply mess up if given the opportunity.
2) All security should be done by the Armed Forces because at least we can rely on them to do the job. It's a last minute panic measure and the soldiers will hate it. Many will have their holiday plans ruined, but reward them all with a huge bonus, free tickets for their families to any event they fancy and guaranteed leave after the Games have finished.