Thanks for the replies.
After returning to the workshop I realised that I had the machine plugged into a plug socket that is on the lighting ring and not into a socket on the power ring.
This ring has a 20amp fuse and when i turned on the machine it did not pop the breaker.
Previously when it was tripping the PSU was connected by only the L N and earth, I had disconnected the tube and taped up the wires with HV insulating tape.
Obviously I want this to be safe, would you still recommend removing the PSU and testing with the separate mains feed?