Hotpoint fridge freezer not cooling, help wanted

I have an older Hotpoint Fridge Freezer Mistral FF92TL

It’s the type where the fridge is cooled by the freezer fan circulating the cool air through the fridge.

I have checked the compressor and it’s running and cooling the freezer.
I have checked the cooling fan by the compressor it is fine.

I have checked the freezer cooling tubes and they are not iced up, so I think the thermistors are fine as the ducts to the fridge are free flowing and not iced over

When I close the fridge door switch the freezer cooling fan works for about 5 seconds and stops. The fan seems to be working itself, but something is telling it to stop?

I have disconected the door light switch and the same thing happens, so I guess the switch is fine.

What next? any clues welcome, thanks