Wow, I've never seen a thread on the Guardian quite like that which developed under my piece this week on the alternative vote.

The monotonicity criterion and a real-life example from the Carnoustie and District ward were both bandied about.

And then there was an ingenious argument about how under AV the total number of votes cast could "sort of" add up to more than 100% of the ballot.

Unfortunately for me, all this highbrow political banter has exposed a small but important flaw in my initial reasoning.

Most of the claims in my original post were correct. In particular, the Yes to AV brigade have to be very careful about the phrase that "every vote counts".

It is also true that rational recourse to tactical voting would not be eliminated by the reform.

But this will require different circumstances from those I was thinking of, and although I was not explicit about the requisite circumstances in the original post, I did make some outright mistakes in below-the-line comments.

I'll be doing fuller Comment is Free pieces on AV before the referendum, but for now – with some nervousness – let me attempt to sum up some of the arguments on this super-cerebral thread.

In regards to my suggestion that Billy Bragg could still find himself voting tactically for the Lib Dems to keep the Tories at bay, some grumbled that as Conservative votes together with Ukip transfers would exceed 50%, the counting of Bragg's tactical Lib Dem vote would make no difference to the result.

In itself, this is not a drop-dead argument, since people can and do vote tactically under first-past-the-post against candidates who command an outright majority, and that is not necessarily dumb in a game of probabilities.

In Cambridgeshire North West, for example, the Tories got an outright majority in 2010, and no individual voter was going to unseat them.

But the exact figures were obviously not knowable in advance, and so a Labour voter might still have rationally plumped for the Lib Dems in order to maximise the chances of an anti-Conservative win.

A Labour identifier living there achieves that goal by seeking to reduce the gap between the top two placed parties, regardless of whether the top one ends up with a strong lead or indeed an overall majority.

The difference with AV, however – and this is the point that I had missed – is that the margin that makes the difference is not between the top two candidates, but between the top candidate and 50%.

This bears on tactical voting because a strictly (and arguably narrowly) rational elector will only vote tactically if there is a chance (however slim) that his or her failure to do so will prove to be the crucial vote that allows a victor whom the elector is concerned to defeat.

In the West Dorset case, let us imagine (as seems plausible) that Ukip transfers are likely to push local MP Oliver Letwin towards the magic 50%.

No individual Labour voter's decision to back the Lib Dems can retard the extent to which this happens, and although they may close the gap between the blues and the yellows, they will not close the gap which matters – that between Letwin and 50%.

Does it follow that Billy will necessarily always be voting Labour in future? Not quite.

For a start, he may be confused, or at least not quite narrowly rational. (After all, if he were that, he might calculate that the chance of his own vote making a difference was so low that he would not even bother to expend the effort in going to the polling station, never mind bothering with tactics!)

For another thing, he may reach a rational political judgment that it is important to close the gap between the blues and the yellows – if the media focuses on the final-round gap between them, as it may do, then he may want to narrow it, so that the seat is thought of as being "in play" for later elections.

Even if Billy is rational, he may not want to assume that everyone else is, and if he can make the contest seem closer he may boost the morale of the local Lib Dems or encourage the party's HQ to pour more resources into his seat and defeat his Conservative enemy in the future.

Or, yet again, even if Bragg were narrowly rational there may be constituencies where he would vote tactically, but not in the way I said ...

The brighter sparks on the thread have detailed circumstances in which tactical voting can happen under AV, and I am particularly grateful to a brilliant undergraduate at Warwick University, Andrew Normand, who has been corresponding with me about this, and coaching me through the logic.

Tactical voting is no longer so much about backing a plausible winner, as about eliminating a likable loser in order to ensure that the second preferences of his or her supporters get counted.

In a seat where the Tories are on, say, 44%, and both the others are roughly evenly split (say, 28% a piece) Billy might plump for the Lib Dems if he thought that Labour's transfers would shift over to them more or less wholesale, pushing the yellows over the winning post, whereas the yellow team's own second preferences would get be split down the middle and allow the Tories to win.

By "betraying" his own party on his first preferences, in this AV world Billy might therefore hope to see to it that they are pushed into third place – that is, to see to Labour so squarely defeated that its second preferences come into play.

It probably wouldn't happen in many English seats, but such scheming is more likely where there are more than three parties, and where there is real doubt about who will get through to the all-important final round.

But with a bit of Ukip surge, and a touch of realignment on European policies perhaps, one can fantasise about some real antics in England as well.

Imagine that Labour reverted to its Eurosceptic ways of the past, so that it becomes well-placed to pick up Ukip transfers.

Imagine further that the Lib Dems have fallen back while both Labour and Ukip have surged ahead so that the battle for second place in West Dorset is a genuine one.

Even so, Billy might still be despairing about the prospects of Labour ultimately triumphing – too many yellows, he may reckon, would peel away to the blues in the event of a straight Labour-Conservative split.

In that event, he needs to avoid Ukip being knocked out so that it does not give transfers to Labour and push them through into a fight that they cannot win.

The response? Vote Ukip, to keep them in the game, knock Labour out – and push its support towards the Lib Dems.

We are, as you will no doubt be detecting, moving deeper and deeper into fantasy waters.

Before the referendum I shall try to do another piece on how many seats there really are where such shenanigans are possible. I suspect they are few and far between.

But for the moment, while the "every vote counts" mantra is dodgy, and while tactical voting will not be entirely eliminated, I have to concede that in Billy Bragg's own case it is unlikely to be a particularly strong impulse.

Perhaps he will soon be singing about first preferences that are as "wild and free" as sexuality.