Should add the noise is coming from the stepper drivers as it only happens when the stepper is moving, so basically the EMI filter in the 24v smps is filtering what comes into it and coupling it to the 24v DC output which causes the DC output to float relative to mains earth because the neutral and earth are coupled at the electricity company's box in the street. I will fit an EMI filter in front of the toroidal stepper PSU to suppress noise returning back up the supply and into the 24v SMPS (and the rest of the house).