I have again today, removed every single plug in appliance & switched off all fixed appliances in the property. Both lighting circuits were also turned off, the noise was still there, no difference at all.
You should try shutting off the mains to the structure. That's the only way to know for sure you've eliminated all the possible sources in the home. There's the chance you missed unplugging a hidden wall wart, or the noise source is from something that's hardwired to the circuit.