I have tried another wall mount power supply but with no improvement, I also switched some cameras just to see if the problem carried over but my guess would be groundloop. I was thinking of trying some groundloop isolators like these https://www.dtsdigitalcctv.co.uk/proddetail.asp?prod=CON701.
Never used them before but I can't think what else might be causing it, I've ran out of ideas so maybe just clutching at straws. Any other ideas welcome.