Hi Norbert,
It's not your fault for not asking earlier
I've learned something new too – I had always assumed 'opto' ESCs (which are generally ESCs rated for high cell-count operation) were not able to power the FET-driving electronics from the flight battery (as it was too large a voltage drop to cater with onboard) and therefore any 'opto' ESCs needed to be fed with 5V. It would seem I've been mistaken
The new Hobbywing ESCs are the same (they function without the red wire connected), so I will just snip the red wires before they go into the servo connectors.
Thanks for your help David