Whirlpool fridge ARC1738 works, but will not defrost

This is an ARC1738 fridge 1.5m tall. It is 4 years old, and now the inside rear wall is frosting up, and not defrosting as it used to.

So far, it appeared that the thermostat was broken as the motor keeps running no matter which setting it is rotated to. So replacement genuine whirlpool thermostat was fitted. But made no difference.

The next suggestion is that apparently Whirlpool fridges are known for insulation breakdown, but have no idea why how or where to check or fix.

The ice builds up at the top and extends down over a day and a half to about 18 ins, it is thickest at the top.

There doesn’t seem to be much else to check, if you have any ideas, I would be grateful.