Thanks Venk and Transpac - given UA's stated policy on Venk's link I think they are being fair. Some of it does still tend to depend on the agent you are using to make changes though which worries my slightly.

I would personally be quite flexible if I missed a flight and had to look at an alternative routing or even date (we always build in a bit of slack on the way home anways in case we have the opportunity to bump and get some vouchers! Tip: never schedule an important meeting for the day after you are SUPPOSED to be home!)