Miele KDN12823S Freezer not working properly

Hi
I have a Miele Fridge/Freezer

Miele KDN12823S
Serial 96162803

Initially the Freezer compartment was completely frozen up
The alarm sounds every morning until its reset. We called a fridge engineer who replaced the main pcb thinking that was the fault, he defrosted the fridge and installed the new pcb which didn’t work at all, this just illuminated all the lights on at the top of the fridge..He then put the old pcb back in and said it was something else and not an economic repair….
I had another look and noticed a large gap around the door where the door was incorrectly fitted. I have fixed this and now there is no gap, The magnetic seal is also intact with no tears, the freezer now freezes to -20, the fan works when you open the door…but then after 24-36 hours the alarm goes off again and the internal temperature is between 0 and -10 degrees….
I have taken the inside cover off the evaporator and there is no thick ice on the evaporator blocking the flow of air from the fan. So I assume the defrost heater is working..If I leave the cover off and the door open the evaporator starts to freeze up as it should.
The Fridge section is unaffected and works normally.
Could this be a thermostat fault…..I am at a loss…
Any ideas would be very much welcome

Thanks in advance