Why were the lib-dems negotiating on the basis of AV if they had been told at previous party meetings it was not acceptable? Or were they negotiating on that basis precisely because they'd been told at those meetings that it was acceptable? Either way, someones being massively devious or dishonest here.
In negotiation you start off with your template and then go for your targets.
Start with AV, on the understanding that it's a prerequisite and then screw it higher into what you really want.
If they don't budge, you consult and come again.
Otherwise if you don't even have AV as your prerequisite when negotiating with the Tories, you certainly are not going to get anything more.