I suspect, as supplied, all the switch units respond to channel 1, thus when we set one at channel 3 for instance it still responds to channel 1. 
Great use as time switches for fish tanks, surprised more people don't use them....advantages as far as I am concerned are:
- Can put 4 switched sockets next to each other on a 4 way extension block (unlike normal digital timers).
- Can change on/off times of the devices from comfort of your arm chair.
- Easy to move clock forward/backward for all switched sockets in spring/autumn at clock change time.
- As you state, need to turn light on to have a morning inspection, no need to faff around in electrics cupboard to locate the timer override switch. Just press "on" and "off" when done.
Great use as time switches for fish tanks, surprised more people don't use them....advantages as far as I am concerned are:
- Can put 4 switched sockets next to each other on a 4 way extension block (unlike normal digital timers).
- Can change on/off times of the devices from comfort of your arm chair.
- Easy to move clock forward/backward for all switched sockets in spring/autumn at clock change time.
- As you state, need to turn light on to have a morning inspection, no need to faff around in electrics cupboard to locate the timer override switch. Just press "on" and "off" when done.