but I think that if we can get AV onto the statute books, it'll be easier to move towards proper PR in the future.  If the 'No' campaign wins, I can't see another attempt to change the system for another 40 or 50 years.

I'm not convinced by this argument.  Although I have to admit that there's a chance that AV will move us closer to a fair system, I think it's more likely that people will say "well we've changed the electoral system so we don't need another change", and I think, if AV is rejected, then the deficiencies of the current system will still be starkly apparent and it will be easier to argue that we weren't given the right choice this time and need to actually be asked about a truly proportional system.

Also, the people most likely to keep banging the drum for fairer votes, the Liberal Democrats, will, if AV comes in, do very nicely out of it thank you, and will consistently hold the balance of power, and I wonder how much incentive there will be to push for a further change to the voting system.

So I think that, probably, a 'No' vote this time will hold more chance for a truly democratic system of voting in the future.