High EMF from electric wiring
That's a big job.  I had to sort out a bunch of wiring in an old factory which had a lot of neutral problems.  I shut off service to the building and, at the panel,  "pulled off" one hot and neutral off each circuit and did a series of diagnostics, checking for resistance between it and the rest of the neutrals, and resistance between the hot and neutral, and hot and other neutrals - alternating whether night-lights were plugged in or not.  If anything seemed out of the ordinary I followed it to the next junction box and carefully troubleshot, and the next, and so on until it tested clean.  (this was a real mess as many circuits were MWBCs.)  I don't recommend this, but I also had a 120v extension cord coming from another building, and connected the pulled-off circuit to that, and sniffed for voltage being anywhere it should not.  I would not recommend doing that with 230v, that stuff bites hard.  

It can help to install a GFCI breaker (UK: RCD) in your panel, as they will trip if there is any cross-neutral activity between circuits.  You can move it around from circuit to circuit. Likewise you can put GFCI outlets on any point in the branch circuit, which will protect everything downstream.  In fact, many service panels have an outlet right next to the panel, and you could install a GFCI in there, attach a hot and neutral wire to the "load" side, and use that to test each circuit in turn.  I wish I had thought of that in hindsight!