Mark – my words
Matthew Rees claims in his blog that AV means Additional Votes (for some). He’s wrong, because he is failing to make the critical distinction between votes, which result in a candidate being elected as MP, and preferences, which guide the returning officer towards the selection of the candidate who wins.
If you indicate preferences 1st for Green, then 2nd for Labour, when your vote transfers from Green to Labour it becomes a Labour vote for the purposes of electing the MP. The Green vote that was your first preference was eliminated, along with the candidate, though the preference remains.
Two parties could claim to have your support, based on your preferences, but not on your final vote (though we might want to question what use support is to a party when their candidate has been eliminated from the race). The Green party can legitimately claim that they had X first preferences, or even that they had that many votes before they were eliminated, but the fact remains that they were eliminated, and those votes all went to other parties, or dropped out of the reckoning because some voters expressed no further preferences.
In summary, your preferences may very well have twice as much effect on the claims that parties make about the levels of support they enjoy in the electorate as a Tory voter’s single preference, but in terms of the effect on electing the MP, your vote counted for exactly the same as the Tory voter’s – one vote, no more, no less.