Its probably best not to interfere...
Going back to the golden rule...treat as you'd like to be treated. Im pretty sure you wouldn't want someone else who knew full well what was going on, trying to break you up if you were in the same position.
However, like I said, and like your flatmates said, "she likes you."
You don't just go getting all touchy without feeling some sense of attraction. Just because she is in a relationship doesn't mean some sort of magical chemical trigger crops up and bars feelings for other people.
It doesn't necessarily mean she will leave him for you. Rather it means, she also likes you, but is currently in a relationship.
If she ever breaks up, it is possible she may try to get with you, somewhere down the road.