Now, apparently, we know.
When new companion Bill, shaken after a nerve-wracking ordeal, asks "Can I use the toilet?" the Doctor is at first taken aback.
But when she insists quite reasonably, "I’ve had a fright, I need the toilet", he points out one of the exits from the Tardis console room and tells her:
"It’s down there. First right, second left, past the macaroon dispenser."