There have been indeed suggestions that the Labour negotiators offered a deal with immediate legislation to provide AV, and a referendum on PR. These may or may not be correct - they have been denied by some people - but in any case the problem was that there was every reason to think that it would not have been possible to deliver any such deal.

All it would have taken would have been to have a couple of Labour backbenchers vote against both offers in parliament - and several Labour backbenchers, and a couple of ministers, had indicated that they would do precisely that.

The numbers just didn't add up to make a coalition of LibDems with Labour possible.

Moreover, even a promise of a referendum would not mean that it would be accurate to say "Labour claimed to be in favour of PR" - after all the Tories have agreed to a referendum on AV, but that doesn't mean they won't campaign for a No vote when it comes. The same would undoubtedly have been the case with Labour and a PR referendum - and for that matter when the AV referendum does come it is certain that there will be Labour people campaigning for a No vote.

All very messy. The LibDems would have been much better advised to avoid going into coalition, but try for a minority Tory government, with some lesser degree of cooperation, in exchange for the referendum on AV. Perhaps they did, and the Tories wouldn't play.