I dont think there is a shift as such. More a left with a louder voice/a chance to get people to listen to their voice. A large minority of left voters who dont feel represented that finally have a voice in JC. There is still a middle ground. JC has reengaged many left voters who have felt disillusioned by the modern politics, and the right wing New Labour. Though the majority are still centre.
Labour have a place to represent the working class, I don't think they can win elections with a 2 party system if they go to the left. It needs the Libs to take back the centre ground and to take some of the votes from Tory and some Labour votes to get a 3 party system, Labour could then get the largest share of the vote, but without a stronger LibDem party Labour wont be able to hold power of they do shift to the left.