The Prime Minister said his government is launching a 'rehabilitation revolution', aiming for more people properly punished but fewer of them returning to prison.
It's not rocket science but has eluded previous governments, he said.
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Damon Smith from Newton Abbot was found guilty of planting a home-made bomb on the London Underground in October.
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Megan Hurley had reportedly been at the concert with her brother Bradley, 21, who remains in hospital after the terror attack.