Perhaps the mel cloud data somehow 'calculates' the consumed energy figure? If the heatpump is oversized could the 'consumed' figure be showing roughly half of what the ASHP actually used? Just thinking out loud.

The hot water settings are:
Max temp: 50deg
Max temp drop: 10deg
Max operation time: 60min
Mode restriction: 30min

(These might not be the same settings that resulted in the mel cloud report posted previously because the engineer may have changed them)

I don't know why an oversized ASHP would mess up Melcloud reporting.  I think the Melcloud consumption is measured; it's the delivered that's estimated. 

Have you looked at the consumption on the FTC control panel rather than Melcloud?  If so, is it the same?  Melcloud does need a decent internet connection and it was down for a lot of users for quite a long time (weeks) late last year.  

You can correct me if I am wrong, but I suspect that your system has been set up to operate as if a fossil fuel boiler had been installed rather than a heat pump.

I think that the system will probably operate in the following manner.

When the upstairs temperature falls below the setting of the thermostat, the heat pump and the primary water pump will be started. A three port valve should also divert the water to the radiators.

If any of the downstairs thermostats are calling for heat, the appropriate solenoid valve will be opened, the relevant zone water pump will be started, and the associated zone valve will be opened.

When the upstairs thermostat reaches the desired temperature the heat pump and primary water pump will stop. If this occurs before the downstairs thermostat has reached the desired temperature then the UFH system will continue to operate, but without any thermal energy being supplied by the heat pump.

Please confirm if my assumptions are correct.

Although I'm often using so little that it hardly seems worth the trouble to actually download the data into a spreadsheet.
Here's a screenshot of my electricity meter yesterday (17aug23)

Yikes... whatever did I do between 11:00 and 11:30 to consume a whopping 0.27kWh ?
That's cost me 8.2p 😥 

We cannot view our daily / monthly / yearly usage with bitish gas

There are 3rd-party services, such as the Bright App from Hildebrand, which you can use to retrieve all the half-hour (HH) usage readings for the past 13 months.
You can download it directly to your smartphone from Playstore or Apple Store.

All UK Smart Meters are required to retain the last 13months readings.
When you first use Bright, you give permission for them to retrieve that data on your behalf from DCC (Data Communications Company).

Beware that there is also an Energy Supplier called Bright. They have no connection with the Bright App.

@derek-m We did receive a handover pack but we did not receive any 'real' education on how to use the heating system - we were shown how to use the room stats and that's about it.

None of the rooms are hard to keep warm, the house is always on the warm side and is very well insulated. We are located in the Midlands and the stats are always well above 20deg until winter kicks in (room stat next to me currently shows 24).

All of the rads have TRVs.

Please continue with your assessment :)

@kev-m It's just the downstairs that is 180sqm (it might be closer to 200sqm), the total is around 380sqm.

We are using roughly 14,000kwh per year, nearly double the MEL cloud 'consumption' number of 7892kwh, I cannot believe that it is something unrelated to the ASHP system causing that usage.

I'm assuming you don't have a hot tub or swimming pool or EV you haven't mentioned and it is the case that your AHSP is using most of the 14,000 kWh.  If that's the case, Melcoud is under-reporting ASHP energy usage.  That's a different issue than a poor performing ASHP and nothing to do with pipe size or mixing valves. Did the Mitsubishi engineer comment on why this might be? If not can you ask them what they think? Maybe something in the area of immersion and booster heaters kicking in when they shouldn't? 

What is your HW set at btw?

I would suggest that you ask for a copy of the full heat calculations for your home.

Do you have a Smart meter? If so, can you look back at total energy usage over the past year and post details.

Do you have any power hungry devices? Anything that heats mainly.

Do you know if there is a back-up heater installed within your heating system? I would suggest that you post close-up photo's of the manufacturer's nameplates for all the equipment.