There is a well sourced rumour that in view of the national interest etc, the Lib Dems will accept Cabinet places without an agreement on having a referendum on PR.

It would be an extraordinary failure of judgement if they do. The Tories merely need say they will let the people decide. The Lib Dems will be finished as a radical force if Nick Clegg agrees to abandon a commitment to a referendum in exchange for office. 

I'm told that he has the backing of the necessary 75 per cent of MPs, which means he will not have to take the issue to the wider party.