The hack to separate the heating and hot water modes created another unexpected issue.
When the heatpumps internal thermistor reads high(not sure of the exact temp but 50C+ sounds right) it no longer turns on the circulation pump every 15 mins to check the water temp.
I realised this had 2 purposes, to check the water temp in the house/cylinder but also to prevent the heatpumps heat exchanger from freezing! The water that runs through the heatpump is just water, not antifreeze or such.
Below graph shows original mode of operation in first half and revised (dangerous) operation in second half. (Looking at HP_LPM, this is water flow through the heatpump)
Cue another software change to pulse the changover relay for a couple of seconds every 15mins if in heating mode, buffer is at temp and outside temp is 5C or lower.
Steve