Bought the Ecogogo VFD. 5.5kw to run 3.9kw motors.

Patched it into the main switchgear, direct onto the motor selector switch. (It's a combination machine with 3x motors). Wired the motors to permenant Star at the switchgear, so only 3 wires to join.

Only question I had was about earthing the VFD, as there was no dedicated terminal, so earthed the machine and the VFD heatsink to mains earth.

Runs great out of the box and didn't cost too much. £190 all in, for the 380v output version.

Will see how it handles load soon.

Thanks for all the advice.