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Thanks for the heads-up, I wasn't aware of the script and having had a look I am left wondering why I would use it for a DietPi installation as DietPi has it's own installation script and I was only interested in Mosquitto and Node-Red and both of those are included in the dietPi software_install. I can see how this script would make it easier if you wanted a full blown Raspbian Jessie install but DietPi seems to be perfect for my application, which is never going to develop beyond a quite basic system. Thanks anyway.
I notice that the OrangePi is not directly supported on the DietPi site anymore, instead they have downloads available for the NanoPi range. As an exercise I downloaded the image for the NanoPi Neo and tried it on a OrangePi Zero 256MB (H2 core) and it works fine. The OrangePi's are generally cheaper than the NanoPi's :-)