After a lot of looking I've bought "Duo Petzl 14 LED on Vertex Helmet", on the basis of having a small head and this being available in a small size. The helmet fits me and my 10 year old daughter!
As with all modern helmets it feels very strange to start with, but you soon get used to it. Modern helmets have foam inserts in the helmet instead of the classic cradle (and there is no space for your Mars Bar!). Consequently it sits high on the head (think Laurel and Hardy bowler hats).
I've had no problems with the thumb-wheel adjustment at the back.
I have 12 helmets of varying types and I've tended to pick this one up for my use every time lately - but it is the only one with a built in light, so that may be swaying me a little.