Ok, after building it on a breadboard and messing with it all day I really appreciate how useful some extra diodes and capacitors can be...
The led which was not supposed to come on on either side was glowing a bit, also ran into serious issues when using different colour led's, maybe because they're very old. After messing about with pullup resistors, generally experimenting with different values and alternate transistors, I put diodes in series with the bases of the transistors to improve the switching, which worked great when poked voltages onto the emitters direct from the psu. However trying it with the actual source voltages brought me back to the original issue with the off led being Mr Glowey. Eventually realised that because the priority switch is dependent on vpp, which is slow to come up, the off side had time to establish a bit of current before the on side was able to block it. Tried capacitors in parallel with the extra diodes, which cured that problem. So it works nicely now.
Because vpp is sourced via a 15k resistor from a 45v supply (it's just how the booster ended up) with a 13v zener, I needed some current gain, hence the addition of a 3rd transistor.
Suddenly Chris's flip-flop looks a lot more attractive!