Ecodan - low supply temperature

Good morning. My first post. I am a retired Chartered engineer.
We have a 7 year old ecodan with underfloor heating. It runs only at night on off peak electricity, and heats the house to 21C except when external ambient falls below-3C (when we run it a little longer in the day). The house loses 1-2C during the day. By observation, the supply and return temperatures differ by about 6C, eg supply 32C, return 26C. All good for 6 years until now.
We had refrigerant leak last year (cracked pipe), which was brazed by the local well respected firm, and the system re-charged. All good.
Lately system output has been falling though everything seems to be working normally at the outside unit. No error codes or leaks. Technician has this week confirmed a full refrigerant charge. Difference between supply and return is about 4C, so that by running non stop we can maintain but not increase internal ambient temp.Maximum supply temp now seems to be 28C at 6C external ambient - we used to see 34C at this external ambient.
Hot water set point is 58C. System used to achieve this easily in two programmed time slots which interrupt the heating mode. However, in hot water mode, supply temp does not exceed 50C.
Something is clearly wrong, but we are stumped as to cause. Any advice would be much appreciated

Hello halfpenny.
symptoms tend to indicate low system charge, probably only a few grams, but critical at low ambient s. The only real check would be to recover system charge and weigh it and compare to stated liquid charge for your system.
Potentially another leak, or under charged previously.

Thanks for the reply. Current ambient is 8C at night ( quite mild for us in January). Discharge temp is stuck at 28C. I tend to think the system is merely picking up residual heat from the compressor casing, and not generating heat as such. Still looking for guidance/clues.

The discharge temperature directly relates to discharge pressure out of compressor. All the compressor is doing is moving heat around, low system charge equals lower vapor flow through compressor and lower discharge hence lower temperature, less drawn power to add to mix.

Unfortunately, there are no system displays to give pressures, only temperatures at various locations in the system and around the compressor. I guess I will have to press the technician on refrigerant charge data.