Does it matter which N goes where? ChatGPT says:
No, it doesn't matter which blue (neutral) wire you connect to N IN and N OUT as long as:
One blue wire is connected to the neutral supply (mains) at N IN.
The other blue wire is connected to the neutral going to the load (appliance or light)...
I only have 1 immersion heater, at least this is what is was when I bought the property... It doesn't look like there was a second one or any other device. The extractor fan has its own switch.