electrical - can someone id what this thing is (on a CU)?

Google for the manufacturer/model ORBIS/UNO on the front of the unit and it's probably a timeswitch.

It's a Din Rail mounted time switch.

Your friend will need to take the front cover off the CU to get a model no. It looks like a 12 hour timer with 15 minute settings.

A quick google shows that it is a time switch

Perhaps it could be powering stair-well or parking area lighting?

the frequent on-off-on-off cycle makes me think it might be for security lights ... are there any round-pin sockets in the flat intended for table lamps, which might be connected to a timed circuit to make it appear someone is at home?

The other common use is for a door entry system postman's timer, but that wouldn't have the on-off-on-off cycle.

Owain

Thanks for replies. The flat is in a tower block - so no swimming pool (alas). I'll try see what it's timing on my next visit.