Okay blokes, second update just incase this may help in the future.

Inverter compressor changed with new ordered direct from Mitsi. All existing PCB's including Inverter PCB left on the machine. Left working sweet as a nut! The only other component I changed was the IPM module as the old one looked a bit charred and as if it had got warm, probably due to the old seized pot.

So in my case 4250 fault was not an inverter issue as such, more like a seized inverter compressor number 1.

I nearly ordered the new inverter pack from Mitsi but bulked at the price so risked ordering just the pot!

Result - hope this may help people in the future?