In reply to remus:
Fair point in principle, but in reality how likely is this:
> Imagine a situation where three parties are in contention, but everyone suspects only two parties are in contention.
I think in practice it would usually be pretty clear that there were more than two potential winners, and normal voting practice should be expected.
Of course it would be better still if some sort of effective STV was implemented officially so it wouldn't need to be guessed at, but that boat was well and truly scuppered during the last parliament.