Bizarre Neff Freezer issue, thermistor fault?

hi all

I have a neff integrated fridge freezer (frost free)
model: K9724X4GB/06
FD8606 9000069834

Fridge seems fine

Freezer has been throwing alarms and the LED display shows AL and LO when switched off and on. The freezer was warm. I put in a small temp sensor which showed swings from 1 deg to -19

I have had an engineer out to check the compressor. He says it looks ok and he regassed the fridge/freezer.

The erratic temperature behaviour seems to be still there. It’s hard for me to find a pattern.

I can say that the freezer is capable of reaching temps like -18
Freezer evaporator fan runs, although not all the time, so motor seems ok.
Under the rear cover, the evaporator is frosty.
There are 3 sensors which thanks to the great fault finding PDF I understand control the defrost and temp. So I think that one or more of these could be faulty. The question is which one

IS there any way to further narrow down the fault. I have already spend £132 on regassing and call out so was rather keen to tackle this one myself. I have just repaired a rather difficult shower unit so i’m on a roll 🙂

I attach an image of the rear of the freezer compartment, with sensors marked A,B and out of shot C (defrost initiation thermostat?/NTC?)

Sorry If I haven’t included all pertinent information and many thanks for any possible insights

cheers Alan