Hi Chris, I was thinking that too. I just liked the convenience of ON/OFF button, since some old people using it. Do you think it's possible to replace it with a single gang type wall switch with light? The relay in the unit seems only direct the current from L to terminal 4 upon activation.
We have this type of "mechanical" timer switch to control our central heating, which is Potterton combi boiler. The switch keeps failing after 2-3 years. This is our 3rd replacement now. It starts with central heating turning itself on for no apparent reason, although the switch is in off...