zx99/5si Zanussi fridge freezer

My freezer temperature warning light came on first after pressing it the noise stopped, but the light stays on our engineer said the fan was sticking, so he got the fan working again. This did not solve the problem. He told us to try another engineer who walked in tutting and shaking his head he said the heater was not kicking in to defrost the freezer and the fan did not work as it was clogged with ice. He said he would not have one of these given as the whole circuit board in the top of the fridge compartment will have to be replaced and this part costs at least £150 is he right, I did not feel he had any idea. The freezer is working enough to freeze ice, but the bread is soft and anything that was frozen has stayed hard but not solid. The fridge still keeps cold. Help!!!