Samsung SR-S20 Fridge Freezer, Not Constantly Freezing. HELP

Hi all am new to these forums, so hello.

We have a Samsung SR-S20 American Style Fridge Freezer, which has a problem on the freezer side of the unit. Basically the freezer stops freezing. If you press the quick freeze button the freezer would actually freeze but as soon as the temperature starts to rise the freezer doesn’t kick in again, and so food starts to defrost.

We first thought it was the LCD control panel on the front of the unit, as it was sticking and you had trouble changing the temperatures etc. We had an engineer out who change the whole LCD panel, and was no longer stick, but after we freezed the unit again, it would warm up and the freezer wouldn’t kick in. So the engineer came out again this time he first swapped the sensors over from the fridge to the freezer to see if this helped, it didn’t, he then came back the next day and changed the compressor on the freezer side, he went away and the freezer started to freeze, but yet again in the morning everything had deforested. We got the engineer back 1 final time this time he changed the circulating fan, and yet again this didn’t help.

In the end Samsung gave us a whole new unit. They didn’t take our old unit away and it has been sat in our garage for about 12 months now. We now need the space so I am thinking of sticking it on eBay the way it is, some in the industry will probably be able to get it fixed for cheap so will have a bargin, as I think this was around £900 quid when we bought. I would like to try and fix it so I could ask more for it on eBay, does anyone have any suggestion what the problem might be and a possible fix?