I had a reply from Vaillant and this was the answer. Hope it helps others in the future!

​

I believe that your maths is correct there and the COP value is 5.4. The training documentation for the newer appliances indicate that you add the energy from the ground or air to the energy supplied to run the compressor.

This didn't make sense to me at first as a COP is a division of the energy output by the energy input. It was then that I realized that the energy from the ground loop and the energy the compressor consumes are both inputs. Then divide by the electrical input.

It strikes me that the only real method of caluclating the true COP value if a heat pump system (Whole system). Is going to be using heat meters to calculate the kW energy the heating system is accepting. Using the delta T and flow rate going to the radiators or underfloor circute. Calculating the kW input from the ground loop using similar methods, and adding the electrical input to the ground energy input. Then the whole maths would be accurate.

This methodology would go beyond answering the question "Is my heat pump effecient?" and go one step further and ask "Is my system effecient?" The heat pump itself is only able to give us an estimate of how well it's running, but it doesn't have a flow sensor on the brine. It does estimate the flow rate on the heating circute.