Thanks for all the comments and advice. I'm away from the workshop for about a week so time to think about next steps.
Yes, plug is a GX16-3 and 2 of the female pins were worn away at the split line but not burnt as far as i could see. I suspect the continuous movement had worn them and eventually caused a spark.
I'll also check the cable when I get back and look inside the vfd

Starting to research new vfd and spindle. Any views on whether 3kW spindle is worth going for over the 2.2 to give more torque at low speed?