As Lola said, you've got to be able to prove you were living together with documents such as leases, date stamped post being sent to that address, bills in your name there, etc.
If you don't want the pressure of the fiance visa or getting married before the student visa is up, then applying for the FLR as unmarried partners is probably your best bet. But if you don't have any of the above documentation going yet, you'll need to get started on that now as you must show 2 full years of proven cohabitation. And make sure to allow yourself the time to apply for the FLR before your student visa expires.
Assuming your visa expires in June of 2006, if I were you, I'd want to show proof of living together from May 2004 - May 2006 at the very latest. Ideally you should have it from March or April or 2004.
Does that make sense?