By immersion switch do you mean a double pole switch connected to a dedicated radial circuit from the consumer unit or a fused spur connected to a socket circuit?
You can get timers from lots of places, I used to have this] one from Screwfix.
Its a double pole switch connected to a dedicated radial circuit.
Does that timer you linked mate, replace the switch? As Im wanting to remove the large red immersion switch which is a pain to get to, and replace it with timer.
Water heaters require a double pole isolating switch to enable maintenence on the element/s so the switch can not be removed, but you can still install the timer after the switch so you do not need to use the switch on a regular basis - it's also handy if you are a bit forgetful and find yourself leaving the immersion heater on for long periods until you realise the water is hotter than usual
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