Well I'd call G4S a BPO rather than an SI, but the discontent goes back a long way further than that - the aborted National Programme for IT in the NHS being the biggest in a succession of SI-led, £billions over-budget, years-late, never-worked programmes that sprayed political shrapnel all over various governments.
Francis Maude tells the tale of the first days of the last government when he dragged the top 5 SIs into Cabinet Office and the conversation went along the lines of:
right you've been fleecing us for years, the game changes as of today. You have these major contracts running for years, and they're far too expensive. You can either cut 1/3 off the cost immediately, same service, or you'll never win another HMG contract, what's it to be?
I guess Capita played the long game...