I would look first at the placement of the thermistor measuring the flow temperature. I am going to assume you have an Ecodan cyclinder too, in which case the flow thermistor is held by a clip in the position I have highlighted in red (return thermistor in blue). The generated energy is a calculation of the energy difference between the flow and return temperatures at the flow rate the unit is also measuring.

The problem could be as simple as the thermistor not being clipped in and generating a lower value than than the actual temperature in the pipe. If you have a different cyclinder you'll have to have a good look to track, it might be hidden under insulation though I don;t think you would have the problem then!

The higher generated energy registered for HW could because the thermistor is picking up more heat when the pipework is radiating more heat at the higher flow temps than when you have a cooler heating flow.

Thanks for the advice.  My set up looks a little different as it's more spread out.  I've found 2 likely suspects.  One of which looks like a probe and is held inside a pipe half way up the cylinder.  Held in place by a bung of tissue which would probably help explain the erroneous readings.  I've attached photos below.  1st is close up, 2nd is to show situ on the cylinder.  Not sure about the other sensor but I've added a 3rd pic of what it might be.  Bit dark but best angle I could get.

The last picture is the flow meter, the other two are of the tank thermistor in its pocket. The flow and return thermistors should be on the main pipework from the heat pump, between where it enters the house and where the pipework starts to meet pumps and junctions. I have attached an image of what the thermistors look like

. The copper body is a couple of cm long and is most likely meant to sit in a u-shaped clip soldered along the pipe. It is possible for them to sit in a bit of plumbing - a pocket - that places the thermistor more into the centre of the pipe, similar to the way the tank thermistor pushes into the tank. In either case I think the symptoms suggest at least one of these has come adrift from its proper place.

The thermistor pocket coming adrift from the flow pipe was a fault I experienced shortly after installation.  It manifested itself as the measured return temperature being higher than the measured flow temperature. The sensors on my system were hidden behind the control box which has a hinge on the right hand side and can be easily moved.  Photo shows sensor after moving pipe insulation.

Great thanks.  I'll have another look when I get home.  Couldn't see anything else near the tank but could be hidden behind lagging.  I'll also take a look outside if its light enough.  Least I know what to go back to the installer with.

@noburn 

Found them.  One on the intake and one on the return.  Both are held in place pretty well with cable ties and are under the insulation.  

It's due for first service next month so I'll ask them to check it out.

Thanks for all your help.  Least I know where they are now.