Thanks for the confirmation. The servos may or may not be used in conjunction with the motors but, yes, I plan to use a BEC to power them if I do mix and match. The balance plug of the Lipo is what I had in mind to connect the Arduino to so that the main motor supply can be turned on separately. Fancy suggesting that anyone would connect an external power supply to the 5V output pins ! Who would do such a thing ?Confession - me in a mad moment of not thinking. I can confirm that the USB section of the Uno does not work any more and that the main chip no longer works in another Arduino, but it may be recoverable.
Thanks for your concern, but I am actually going to take the full pack voltage from the balance plug, not just one or 2 cells. The balance plug is just a convenient way of connecting the Arduino whilst the motor controller is attached to the main Lipo plug.