A bit of a weird one here, but hopefully someone might be able to point me in the right direction.
Last year I moved into a house (Ireland) and the following happens intermittently (once/twice per week) - when I flick the switch to turn off the power sockets in two locations in the house (that I know of), it trips the trip switch that controls all the power sockets in the house. Note that it only happens when turned off the switches at the power sockets, and never when turning them on! The two locations are somewhat related in that one room is above another, but sockets are on different walls.
Location #1 - Downstairs 2 power sockets that connect into one recognised-brand surge protector (1x 6 sockets) with 6 devices connected (TV, Surround Sound, Powerline Adapter, Router, PoE for fixed wireless broadband, 8-port network hub) and the satellite TV device connects to the second socket.
Location #2 - Upstairs 2 power sockets with one that connects into a recognised-brand tower surge protector (10 sockets) with 7 sockets connected (computer, 2x monitors, 1x network switch, 1x powerline adapter only really powered on). A lamp is connected to the other socket.
Note: All devices and surge protectors used without issue in last home. When it trips, it is always after the socket was switched off with one of the two power extensions connected.
I used a socket tester to test wiring and it reports all is OK.