Hi I'm John i will help you with the problem.
The defrost timer could be the problem its located in the same control panel as the thermostat you can turn the small cog on the timer to advance it to the defrost cycle if the heater comes on replace the defrost timer.
The timer is number 257 in the breakdown and thats the same location as the thermostat the dial you set the fridge temp, if the heater does come on it will start to thaw the frost on the freezer.
If it frosted up again the part to check is the number 230 its the defrost heater its glass so you can look to see if the coil is broken or the defrost thermostat if the number 230 is ok replace the defrost thermostat.
Its a coil so you can look at it to see if the coil is broken.