Hi Liam, this sounds like a failed temp sensor.
Its detecting when first switched on its too warm so starts the cooling process, then it reaches say -18 in the freezer at which point the sensor stops responding, at this point the fridge is told its still -18 but in reality the fridge is warming back up.
The fridge will be detecting this as a fault since it knows the temp should be fluctuating and hence why the master alarm light is on, this will need an engineer to attend, in order to rule out its not the PCB incorrectly reading the board, but 70% of the time its the sensor at fault.
Should you like me to recommend a suitable engineer in your area please provide me with your postcode (this may be Samsung depending on how easy it is to source spares).
You are on the right tracks, apologies I didnt see the second part to your question.
If the sensors have been replaced it must be the PCB, most likely a faulty relay, if your changing the PCB in the majority of cases you need to ensure your buying a programmed board, as this may be why the previous one didnt work as desired.
The PCB's are rather expensive, my suggestion would be to look to pay for a repair and protect plan which is around £150 from someone like D&G, they will cover the cost of replacing the faulty PCB, and give you a further years warranty, as the challenge with replacing the PCB yourself is that you need the specific engineer manual for the PCB to programme it (as Samsung dont have standard instructions for all their models)
Never heard eSpares say anything other than its a straight swap :-)
If the board you have is pre-programmed the only other component it can be is the compressor which is overheating and failing, but think its unlikely, the way to test it is to monitor the voltage going out of the PCB to the compressor when you turn it on for the first time then the voltage after a few hours and its reach temp (and the compressor has stopped), and then again when the red light comes on if the PCB isnt outputting voltage it shows it must be the PCB.
Thank you for the input, to me it sounds like this is a PCB fault, as its not controlling the defrost element properly.
I would certainly be inclined to go for a repair and protect option, as this will be a costly repair and very time consuming.