Went around to have a look and found them all sitting in the bamboo at the corner of the house creating a cacophony. Looked at the roof and realised what was going on:
It's a shima-hebi or Japanese striped snake. Not poisonous so not that much of a worry.
I was rather disturbed to see it on the roof, though. We still don't have flyscreens on all the windows here and we don't leave the downstairs windows open too much for fear of uninvited cats coming in for a look see but I had assumed the upstairs was pretty safe and routinely leave all the bedroom windows open.
I have no problem with snakes- they eat mice, do their thing and don't bother me if I don't bother them- but I do not want to find one curled up on my pillow or among Amy's menagerie of soft toys!