Visiting my girlfriend in the UK, What documents do I need to add to my application ?

Ive been living in the UK for a year and a half, on a Tier 4 student visa. I met my girlfriend there and we moved in together. She's lived with me ever since and for 6 moths I've moved in with her family. Now I'm back in India and I plan to visit her for 6 months, at the end of which she plans to come home with me. What documents do I need to put in for my application. Right now I have documents for my income , passport and a latter stating how long we've live together. would that be enough ? thank you very much for your advice in advance :)