I would like to install a boost switch on my water cylinder's immersion heater, allowing me to put it on for just 30 mins or an hour. However I'd also like the flexibility to turn it on for longer with a normal switch (20A DP). Could I wire them like in this diagram?
It is normal to have a single switch isolation point and what you show could lead to problems where someone thinks it is isolated but it's not and the timer could cut in making it live even when proved dead.
It would be normal to have isolator and timer in series and for the timer to include an override button. In that way it will auto revert back to timed system after.