It's just based on your income, they can give you a 6 month one if your income's borderline or a 12 month one if its low. I was on a £3300 loan + £200 a month from my parents and got a 12 month form with no bother at all. You just have to send them your student loan certificate (it says in the HC1 which bit of paper) and the HC1 and they'll post the certificate and the HC2 out within about 2-3 weeks. It only covers certain dental treatments and you usually have to pay the first £15 for a check up, then £30, £50 or £100 depending on what else you have done.
Hope that helps.