How to fix a I have a Beko CDA543FS Frost free Fridge/Freezer not freezing?

I have a I have a Beko CDA543FS Frost free Fridge/Freezer and it has started to not freeze properly. I found that the freezer was defrosting and the fan was only turning for a few seconds and then stopping. After a few more seconds the fan would start to turn and then stop again.
After leaving the fridge off for a day to let it defrost properly I turned it back on again and it appeared to work ok for a few hours. The freezer compartment was getting icy cold but I went to it later and the fan was starting the switching on and off cycle again and the freezer was getting warm again.
Any ideas as to what the problem is? and it has started to not freeze properly. I found that the freezer was defrosting and the fan was only turning for a few seconds and then stopping. After a few more seconds the fan would start to turn and then stop again.
After leaving the fridge off for a day to let it defrost properly I turned it back on again and it appeared to work ok for a few hours. The freezer compartment was getting icy cold but I went to it later and the fan was starting the switching on and off cycle again and the freezer was getting warm again.
Any ideas as to what the problem is?