Update: The system is now running as expected, even with the indoor temp sensor enabled.
So the most important setting here was setting the pump to ‘auto’ instead of ‘intermittent’ which is key for an open-loop system without a secondary pump, and the realisation that if the system has been off for a while the degree minutes will be negative therefore flow temp will be increased temporarily to compensate. It may take a while for the algorithm to sort itself out.
Thanks again for your help.