When Amy first met her, she said something like "you're so his wife" and she replied, "Ooh, you're good, I'm not saying you're right, but you're good".
I can't see a reason for them not to marry 'properly' now though, they obviously want to! I love all the scenes where they flirt, they obviously have (I hate saying this, it sounds so wanky!) great chemistry.
It made me laugh in Confidential when she was saying about if all Tesselectors could kiss like that.