Sounds like you have some leakage to Earth.
 Does this happen with the machine disconnected from the phase converter and just running the phase converter?
Hood

I can run the inverter running with or without the machine connected without issues. It's only when I run g code that I trip the rccd. Strangely I can jog the machine without issues??

Would it be worth creating a program which just worked each axis at a time to narrow down the cause.

You could try that and see but why it trips the RCD in the house I am unclear as I would have thought it must be an earth leakage for that to trip.

Hood

At the moment the house and the workshop share the same earth. Would it be better for each to have there own earth?
Any ideas how to trace the earth leakage?

Not really as I am not a spark but what I would likely do is try and remove all circuits and enable one at a time and see when/if the fault occurs.
I would look at the wiring to see if there is some damage to insulation which may allow the current to leak to earth, something like a nipped cable or insulation that has been cut by swarf. It may be hard to find, especially if the problem is in multicore cable as you may not see any damage from the outside but if you can narrow the problem section of wiring down to a specific area it will be easier to find.

 Hood

Also maybe flexing any wiring that normally gets flexed with axis movement may show up the fault.
Hood

I've narrowed the fault down to the x axis. (It would be the motor hardest to get to) The motor doesn't move when the machine is in operation. But I will give everything a good shake while testing. I've checked each terminal of the motor to earth, and there is no obvious fault. Maybe I'll swap a motor from another axis and see how in get on??

Are there any other checks I can do ok the motor?

How easy would it be to swap drives?

If you have a megger you could test out the motors insulation, make sure its disconnected first
Hood

How easy would it be to swap drives?

If you have a megger you could test out the motors insulation, make sure its disconnected first
Hood

What's a megger? I have a multi meter and that's about it, I could swap drives, but motor would be easier.

A megger is used to test insulation, it puts a very high voltage through the wires, mine has 250/500/1000v settings.

Well swapping motors would let you know if its the motor, if it still happens then may be the drive or the wiring.
All trial and error I am afraid
Hood