I've covered this before, but I'll do it again, now it's in the shop window.
I quite like the Alternative Vote and in the event of a referendum, I'd be in the yes category, but clearly, one concern is that it isn't proportional. I think there's a way to address that which might be acceptable across the political spectrum - PR for the Lords. All the major Westminster parties back election for the Lords (at least in part). All it would take is a repeal of the Parliament Act and STV or open lists for the Lords and you would have a proportional house with an absolute power of veto over legislation.
I think it's a viable compromise. Labour have flirted with AV for the commons and both open lists and STV for the Lords. The Lib Dems want STV for the Lords and seem prepared to tolerate AV for the commons. The Tories are prepared to accept a referendum on AV and I can't see them objecting too severely to a proportional system in the Lords. I don't imagine too many smaller parties or independents would have serious objections either.
The only sticking point could be repealing the Parliament Act, but I see that being almost inevitable if the upper house were to become elected and have democratic legitimacy on a par with the lower house.