Thanks for your help and advice. Yes I'm in contact with Hitec and had done the test direct to the rx at their request but beyond that they haven't had any suggestions to offer me.
A smaller cap (the 500pf/0.5uF) across the power leads does nothing at all, I don't have any larger capacity low-ESR types (like a 220uf 10V tantalum), I have lots of electrolytics (200-400uF/50-60V) because I'm an electric flyer and we use these to resolve back EMF issues on long leads between the battery and ESC (which is what gave me the idea to try it in the first place).