You’ll need some moderate weather when the heat pump has to cycle because it cannot modulate low enough. The fans out the outside unit will turn off and the compressor stops for some time, while the water is still kept circulating, and that’s when you’ll want to look at the temperatures. Alternatively, you could temporarily set a significantly lower target temperature on your controller, e.g. 15°C instead of the typical 20°C. As your system is still running at the required flow temperature for the warmer indoor temperature, it will turn off shortly after the change until the flow temperature drops to its new, lower setpoint. During that time you can read the temperatures off the controller and then we’ll know exactly what your COP error is. You can turn the desired temperature right back up afterwards, just check the fans are turned off while you still see a nonzero flow rate. Allow a few minutes for equilibrium to settle in and then read the temperatures.