the one which visited our offices recently but a bit more mobile. We called a lad from down the road to identify and deal with it. Unfortunately, when we opened the door again, it was just disappearing into a gap in the roof which it must have reached by climbing onto the basin and then springing 100cm to the roof. Luckily we found it again in the laundry room and managed to frighten it off.
Turns out it was another rat snake and I'm wondering if it was tracking the rat through sense of smell, perhaps from the droppings. Whatever, I'm keeping my door firmly shut from now on. I have to admit that at the moment I'm quite wary of things which could be lurking in corners!