Thanks for the comments.
Tablets vary a lot in price. I use an A4 working area NISIS pad, which cost £70 or thereabouts. It works fine for me, but I'm not really very used to the world of graphics tablets. Most people say that WACOM ones are the best, but they're far more expensive. Personally I've always found past a basic point you can get the same effect with all artists materials, the expensive ones are just there because a lot of people will buy the most expensive by default. Provided it has a good range of pressure sensitivity and a good returns policy, I'd pick up a relatively cheap one.
The main mistake I made in buying a tablet was buying one that was larger than I really needed - if I was buying another I'd probably just get an A5 one. Because most of the work I do on a tablet is relatively tight, it's mostly done zoomed in. The only reason to have an A4 (or larger tablet) would be if the single marks you were making were particularly large - 20cm+. Most the marks I end up making on a tablet are more like 0-5cm.
I've spent a few more hours fiddling to try and capture the expression/likeness better, and playing around with the brush effects to try and get used to using a tablet. To try and get a better likeness, I firstly made a copy of my work (I'm a coward), then went about copy/pasting areas of the painting layer, and free transforming them until they were better positioned. I then worked back into them with a brush.
Most parts have had some subtle tweaking, the main areas that I've changed are:
1) Bringing the right side of her face in (lhs of the painting) as it was too wide
2) Lowering both eyebrows, as they were too arched
3) Making the eyes slightly larger
4) Making the teeth wider (thanks Bernadette)
5) Reducing the height of the hair, it was too big
6) widening the forehead slightly on the left temple
7) Widening the bridge of the nose slightly
8) Lowering the smile creases on both sides (they were both far too high)
9) Bringing the left cheek out wider
10) General work on the skin tone
The main things I'm still not happy with are:
1) The eyes seem slightly small - I don't think they technically are, but something is giving the effect
2) The skin tone
3) Some of the lines are too dark
4) The hair
5) The eyebrows (they look a bit like they've been drawn on with marker pen)