Scandinova ff6745d Fridge/Freezer evaporator fan not switch

Hi there, I have a scandinova ff6745d dual compressor fridge freezer which has an issue with the freezer section. I went on holiday for 3 weeks and on returning I found the door slightly ajar and all iced up. After defrosting, the bottom of the freezer is at the correct temperature (-18degC) but the top of the freezer isn’t cool enough. Everything seems to work other than the evaporator fan doesn’t turn on with the compressor i.e. bottom of freezer gets to the right temperature and it auto defrosts itself. The evaporator fan spins freely and if I put the fast frost button on the evaporator fan works fine and the freezer gets to the right temperature from top to bottom.

As I don’t have a schematic diagram I’m unsure why the evaporator fan is not switching on in normal operation as my understanding is that when the freezer compressor is switched on the fan should switch on too. I replaced the freezer defrost timer (£5) thinking this could be a possible cause but it wasn’t and I’ve subsequently read up more and think its not related. The relay seems to be operating Ok to switch on the compressor.

So my question is; should the evaporator fan always switch on with the compressor and is there anything that might stop this happening? If anybody has a schematic diagram I could hopefully work it out myself!