I think you may have hit the nail on the head here Graeme as I've fitted a twine heat exchanger shower unit to a Td5 (Defender) without a problem. I'm not sure if there is any difference between the D2 and Defender but AFAIK the flow diagrams are the same.
I simply replaced the metal section of pipe between the top radiator hose and the heater with the twine heat exchanger - no need to cut any heater hoses and I can easily return everything to standard by refitting the pipe section if necessary. I did need to reshape the turbo heat shield to enable the heat exchanger to sit neatly in place of the pipe.
Unfortunately this is all a bit off topic and not much help in regard to the original enquiry regarding the D3.

