Pulsacoil are made by Gledhill as you correctly point out
Gledhill do a fixed price repair.
https://www.gledhill-response.net/one-off-repairs/
In its simplest explanation it is a hot water cyl at low pressure (open vented) heated electrically. it has a cold water mains pipe running through it. Due to conduction the mains pressure water goes in cold and comes out hot.
There is a train of thought that having it "too hot" saves money. ( it makes you put the plug in the sink and add cold so hot water is not running directly down the drain
).
If the appliance proves to be working correctly and the sensor isn't faulty there is a mixer valve we used to set up and use in establishments where people were vulnerable to being scalded, this can be set to mix the hot and cold water to a temp of your choice before it reaches the tap. Depending where you fit this device it could be for one hot tap or all hot taps.