Thanks Ted, problem solved.
Indeed when I looked inside the P4 there was no earth connection from the PCB to the gain pot on channel 1…. and no evidence there ever was one, I have the pictures to prove it
The difference after the fix is significant, no hum issues at all and just as clean as my Allen heath zed 10fx mixer. In fact P4 does sound slightly more open than the Zed, I’m very pleased.
Many thanks for your advice.
PS the powersupply is 12VAC so no problem there