We are sadly short of cool yarn shops, at least in my bit of London, @Fairislefandango. Although it makes me sad, on balance it is a good thing for my bank account!
I did the gauge swatch for my Maria top yesterday. Pattern says 4mm. I was two stitches short with that, and 1 stitch with 4.5mm, so I am going to start with a 5mm hook.
However, I am deeply skeptical - this always happens to me! My gauge swatches are way off, but when I get started, whatever I am doing is wildly too big. So I'm putting money on much swearing, frogging, and ending up with a 4mm hook in about a week's time. It'll be super annoying this time though, because I'm working in striped rows, so I'll end up having to join all the bits, or I'll likely run our of yarn (which I am half expecting to do anyway).