Ok thanks guys for replies. So as I understand it, so correct me if I am wrong. So any current in a circuit instead of returning down the neutral wire to the fuseboard, if there is a neutral earth short, it could return down the earth wire instead (because ultimately the earth and neutral are joined together before they enter the house) and because that is smaller in size, it potentially couldn’t handle this current and could overheat causing a fire risk?
Oh and btw the electrician has been here this afternoon and found the problem. It took him a couple of hours and he had to take a few light fittings down to work out how the wire ran around the house but by a process of elimination he tracked it down to a faulty light fitting. He showed me on his meter that when he removed it from the lighting circuit it cleared the short. When he took it apart the neutral wire had the insulation damaged on it, where it was shorting to the body of the metal fitting. He even fixed the fitting for us!
So we are all good apart a few C3’s.
Thanks again