A bit more info.  The Cosy App has the Target Temp to reach, and an internal temp measured by the Primary Pod sensor positioned somewhere in the house, just like a traditional thermostat.  The Cosy Controller will use that to turn on/off the heat pump as required, just like a thermostatically controlled system.  This is what seems to be happening even with the Flow Temperature control set to Weather Compensation.  The current values for the WC curve endpoints are 36C for Hot Weather Flow Temperature (at 12c) and 57c for Cold Weather Flow Temperature (at -10c.)  I am led to understand that the WC curve is linear between these two points.  

From discussion elsewhere it would seem that it is currently controlled by the Primary Pod sensed temperature as the Cosy 9 has no problem putting out a flow temperature that can reach the requested 22c.  What I should do is whack that required temperature up to, say, 28c (accurately - a temp the heat pump can't reach), which will prevent the Controller turning the heat pump off.  Then the temperature in the house can be controlled by careful adjustment of the WC end point temperatures.  As the water flow and return temperatures start to reach the dT (is that 5c on a Cosy 9??) then the flow temperature will be maintained by the Heat Pump (based on outside temperature NOT inside temperature), energy usage reduces, and the system stays on, ticking over.  If at that point, the internal temp is 23c I can tweak down/up the WC curve endpoints to bring it closer to the temperature I want.  Clearly requires the system to be properly balanced.

I'm away for a couple of weeks, back in mid-October, so at that point I may well try some fine adjustments based on this approach.  I certainly wish that Octopus would explain this properly in their help because it's quite confusing when you have an expectation in mind and an actuality that is different!  I still have my fingers crossed that I don't have some serious explaining to my wife to do - this whole thing started because our old gas system just couldn't keep the house comfortable.

@andrewj

It sounds to me like you are onto the correct approach. 

If the app/pod acts like a traditional thermostat then the right thing to do is indeed to turn it above the desired level, then use WC to adjust the house temp (balancing radiators if necessary).

Make small changes to WC at say daily intervals, to allow the house to stabilise in between.  It took me a couple of weeks tweaking to get the optimum value but once done its done more or less for good.  You shouldn't have to explain anything to your wife, its just a process of tweaking which, because the house responds slowly, inevitably takes a bit of time.

Posted by: @jamespa

↑

You shouldn't have to explain anything to your wife, its just a process of tweaking which, because the house responds slowly, inevitably takes a bit of time.

It was more of a "Why does this house still have cold rooms when we've just spent £6.5k?" sort of explanation!  She wouldn't be interested in the tweaking.

Posted by: @andrewj

↑

Posted by: @jamespa

↑

You shouldn't have to explain anything to your wife, its just a process of tweaking which, because the house responds slowly, inevitably takes a bit of time.

It was more of a "Why does this house still have cold rooms when we've just spent £6.5k?" sort of explanation!  She wouldn't be interested in the tweaking.

 

Balancing the emitters is the key to that, although the sharing of heat between rooms plus always on philosophy helps also.

Posted by: @jamespa

↑

Balancing the emitters is the key to that, although the sharing of heat between rooms plus always on philosophy helps also.

Actually, I think the issue is the different rate of heat loss of rooms when the system is on/off.  If the heating doesn't stay on long enough to bring a room up to temp or it gets to temp but loses heat too quickly - because the area with the thermostat warms up quickly/cools down slowly - no amount of balancing will work because you just can't maintain enough heat surely.  It's the main reason why I swapped for an always on system and why I will run it without set back: balancing in this scenario, as you have said amongst all your other great advice, is essential to even out any temperature differentials between rooms.