I am renting a 2 bed flat but may need to rent the second room out soon - does anyone know the legalities?

That is technically sub-letting and your tenancy would not allow this. Speak with your landlord and explain the situation and see if you can come to some arrangement. Sharing with a friend would probably not be an issue, he would just need to reissue a contract in both names, but he may be opposed to renting to a total stranger.