Thanks
@Devwon and
@Greatscott , I've arrived at an acceptable solution with the bits I had to hand, the Hey,OK thing looks great (and I've bookmarked his site) but I'm skint

I chose to use Aux 2 channel, switched on and off by switch C, substitute your own values.
1. Go to mixing on the menu. Scroll down to mix 0 and click on it. This will open a screen with master/slave/rate etc.
For Master select Throttle, for Slave select Aux 2, for Rate select -100% +0%, for switch select C (two position [0][1]), for Trim select Inhibit.
2. Go to Aux Assign. For Channel select Aux 2, for Input select Inhibit.
This got me reverse lights, but they only came on with quite a fast reverse. To improve things;
3. Go to Sub Trim. For Aux 2 scroll to +100.
For my nano switch linked to above it is triggered at about 45/100 showing on Aux 2 on the monitor screen. By chucking maximum sub trim at it the resting position of Aux 2 is moved to 29/100. This means the throttle only has to get to about -15 to trigger the reverse lights. Not perfect as the mix only links the reverse throttle and Aux 2 as a mirror of each other, whereas I want ANY reverse to trigger 100% Aux 2.
Stay tuned for the next thrilling instalment... or just buy the Hey,OK brake/reverse/driving light controller!