Last night, around 11.30, a lot of noise of unusual character started coming from the roof. Stepping out onto the balcony, you could hear it from outside, but could not see the part of the roof it was coming from. It sounded like something hitting a tile on the roof.
It persisted, and was loud enough to stop sleep, so after midnight I was carrying the stepladder upstairs and poking my head through the access hole with a torch. A furry movement was noted, and eventually a possum appeared clearly, walking nonchalantly from one part of the roof it appeared to be intend on attaching, to another corner of the roof space. It seemed to me to be trying to make or enlarge an access point through tiles. The sound resumed about 20 minutes later, but did stop.
While rats in our roof space are common (I have already had to bait it once this so-called autumn), and I always assumed it might be next to impossible to block all tile roof entry spaces to prevent rat entry, when you find you have a possum in the ceiling, it's time to call in the professionals.
I have done so already, and will post the results.
Update: the possum man identified a clear gap in some roof flashing where he was sure the possum had entered, and it was at the spot where the possum had been noisily doing something on Wednesday night. He said that the way the roof flashing had been pushed/chewed open, it possibly was finding it hard to get back out. This is consistent with what I had guessed. (He said it is unusual to have a possum in the roof at that time, as they usually leave of an evening to eat, returning in the morning. This would also likely explain the 'walking' sound that we had heard from the roof/ceiling, as it was generally heard at those times.)
Trap cages were left in the roof space last night to try to catch the possum if it was still inside. None caught yet, though.