Thanks Patmat and mfr02,
I originally asked at the hobby shop where I bought this ESC and was told that there was no way out, and that it would be best to use the BEC circuitry and perhaps buy a new receiver.
Not being satisfied with that answer I decided to investigate further and have found this website and forum. I have read a similar post about disconnecting the red wire, and decided to ask for help as I am impressed by the knowledge and help offered.

I am to understand then that the red wire can be a two way route. I know how to carefully remove the red wire connector from this plug.

My reasoning to use a separate power supply for the Rx is that I would maintain control of the boat despite the reduction of the voltage of the main batteries for the engines, and that the power to the radio would not be cut off. The weight of the Rx battery pack will not be noticed in this model.

The final question is whether this one ESC is OK for the two Graupner 700BB motors in parallel.

Thanks again for you help

Reilly4