Hy Alistair. My English is poor and your text is to long for me LOL.. but how I uderstand what you need I dare to recomand ....
here is one.
The amount of water from a heat exchanger usualy is approx. one third from the all amount of water in an installation for heating and hot water.This is important for your heat exchanger coils dimensions.
About thermosiphoning...if you are not using a pump ,in the circuit,than you need to use large diameter tubes let say 3\4 tzoll because of the friction and the heat exchanger have to be in a lower position like all the rest...and don't forget the coils ,I sure is about copper tube and is not 3\4 so I am sure you will need a pump.
I have a simple rocket stove ,for cooking and I use ash for insulation.
I hope I wasn't offtopic LOL
Please excuse my English ,is all I have and I hope is understandable for you.