Jem, I didn't use "the smallest room" cos it would sound twee, and isn't right for my first person vp. I used small room cos it involves her being in there and things like:
"She pressed herself against the back wall of the small room."
She is inside the room for some time, and some action takes place in there (violence actually), so I just wanted a way to describe the room easily.
Loo, toilet and lavatory all mean the same, really - both the porcelain and the room in which it dwells.
Emma, thanks - downstairs loo would work well if it was being said by people who lived there. She doesn't live there and it's an alien environment for her (she is in someone else's house and is attacked).
I really appreciate all the suggestions. Not sure there is a good alternative, so I'll just have to soldier on with what I've got. But nice to know I'm not missing something obvious...
Deb