I had 3 stone on by about 30 weeks but don't seem to be gaining at all now, my weight is the exact same every week. I stopped weighing myself around 20 weeks with DD as my weight gain was depressing me, just before I left for the hospital to have her I checked and I had gained 3 stone 3 pounds. I would try not to worry about it, weight gain can really slow down in the third trimester and I lost all the weight last time. Also, DD was just under 8lbs, average weight.
A friend of mine put on a stone and a half really quickly, but she hasn't gained any weight in the past 8 weeks or so (she's 30 weeks).
Its different for eveyone, I'm gaining weight at about 1 lb per week since the start of the 2nd trimester, but only gained 5lbs in the first trimester.
I eat loads of fruit when I'm peckish, it keeps my mind off of starbars and moros
Don't worry about it too much, everyone is different. Try taking pregnacare or some kind of pregnancy supplement as part of feeling hungry all the time may be that you are lacking in certain vitamins or minerals. I found I wasn't nearly as hungry after taking supplements for a while and the weight gain levelled off.
Aperture I was about the same - 2 stone at 18 weeks. I didn't weigh myself since then till last week and I had only put on half a stone in the meantime so everyone is different. Just eat well and eat for one!!