I think it's a troubled argument, and I think it's a troubled argument for a couple of reasons.
On the facts of the case, I don't think we had an election that was fought over electoral reform. I think it was a long, long way down the list of issues on which votes turned and on which discussion occurred. The particular facts of the election suggest to me that it wasn't one in which there was a lot of discussion.
For the most part, we don't have elections that are typically fought over particular issues. That's the exception, and I think that's a normally defensible way of having elections. We choose leaders and parties and then we evaluate their performance. On the facts of the election, I'm convinced by that argument.
On the second point, I think this is a major institutional change. I'm not sure a convention has emerged that these changes have to be met by a referendum, but it seems to me that because it is such a fundamental change and because self-interest has such a clear potential to contaminate the debate, since parties are talking about the rules under which they'll be elected, perhaps there ought to be more of a check than just parties voting on it now.
To the third point, to be very bold about it, if you'll allow me, I've yet to hear an argument about the incapacity of voters to make a decision during a referendum that doesn't also condemn the decisions they make during elections. That is to say, the simple-minded, manipulable, easily confused voter who apparently won't be able to make a reasoned choice during a referendum is also the voter who elects everyone in the House of Commons. I think it's a dangerous discussion if we start to believe that voters are unable to make informed decisions on fundamental matters.
In sum, I don't think there is a constitutional convention preventing you from having a referendum, but I think that if 60% of voters in the last election were in favour of electoral reform, as seems to be claimed, surely it must be easy to win a referendum in that case.