It's worth reading Boris Johnson's interview with  the Telegraph today, particularly for the passage in which he suggests that he may be open to supporting electoral reform.
Discussing a recent debate with Alan Johnson on proportional representation, he remarks:
Although my side won the debate and I was listening to the arguments, I have to accept that there are arguments that are difficult to despatch very easily. There is an unfairness in the current system. The advantage of first-past-the-post is that it delivers a decisive result. But that very virtue may be disproved. If it turns out that we wanted to kick them out and we didn't, that is a big argument against FPTP.
Boris has recognised a key contradiction in David Cameron's approach to electoral reform. On the one hand, the Tory leader consistently claims that first-past-the-post always produces a decisive result. On the other, he spent much of this week warning of the risk of a hung parliament. These two propositions are not mutually compatible.
Should Cameron wish to make a credible case against electoral reform, he'd better start thinking of some new arguments.