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Nick Robinson's ³ÉÈË¿ìÊÖ documentary on Thursday, '5 Days that Changed Britain' (9pm, ³ÉÈË¿ìÊÖ Two) doesn't help much in solving the mystery of the AV-without-a-referendum promise.

As I explained in my blogs earlier this week, this lay at the heart of the coalition negotiations.

In tonight's programme David Cameron denies misleading Tory MPs, whilst Nick Clegg says it was "an offer that might have been made" by Labour, though not directly to him.

These are the relevant exchanges:

Robinson to Cameron: "Did you mislead your MPs by saying Labour will give them voting reform without a referendum?"

Cameron: "No, because I was absolutely certain in my own mind that was the case. And I had, I think, good reason to be certain...A number of people had told me what was - what they thought was - going on and conversations that were taking place about AV without a referendum.

"And also I'd had a conversation with Nick when I'd argued very vigorously that you couldn't do alternative vote without a referendum - it would be wrong."

And Robinson to Clegg: "But, but if Conservative MPs think they're being mislead into backing a referendum because you told David Cameron: look we can get this changed without one, you're saying that's wrong?"

Clegg: "The perception, which I think was accurate, was discussions are out and it might have been an offer that might have been made and might have been considered. In answer to your direct question was it ever formally made to me, no it wasn't formally made to me."

So where does that leave us? Very confused.

If you put a gun to my head and insisted that I explained what happened, then I do now suspect that in an unguarded moment Gordon Brown (or some other Labour figure) did mention AV without a referendum to Clegg or the Lib Dems, but not - as Clegg says - as a formal offer, just a suggestion perhaps. (Both sides should have known, though, it was an offer Labour probably couldn't deliver, as Labour MPs would never have agreed to it.)

Then, Clegg may have over-egged the position, and the strength of his hand, and what Labour had said, while talking to the Tories. Quite understandable in the circumstances.

And in turn Cameron may have failed to probe with Clegg how strong Labour's 'offer' was, simply because he knew he had to offer something to the Lib Dems on electoral reform, and needed some stick to bring the Conservative Party into line.

All in all, if my theory is true, then it's one of those cases which reflects quite badly on the leaderships of all three parties.

Alternatively, of course - the cock-up version of history - it may just have been a series of terrible misunderstandings.