In reply to J Whittaker:
My feet are size 2.5 but quite high volume. My summer boots are size 4 and my winter boots are size 5. That works for me with the correct socks. I have found that as long as my heels don't lift, having a little extra space in the toe isn't a problem - you just have to make sure that the heel fits well. In my case, the high volume of my feet helps with this - it might be more problematic if you have lower volume feet.
In terms of crampons, I have got a reasonable fit with Petzl Vasak and Sarken for general use. For more technical use I have heard good things about BD Cyborg, althought I haven't tried them personally.
Post edited at 09:18