Whats the temperature of the freezer, compared to what its set to?
Normally the alarm will sound if the door is left open or if it cant maintain a temperature
So the fridge is clearly struggling to get down to its desired temp of -22. Freezers should be working at below -15.
Though nothing has defrosted a 13 degree difference is substantial, these type of faults would normally suggest a door seal issue (cold air escaping), however if your not see alot of frost in and around the freezer its unlikely to be the culprit.
Next would be the thermostat, if its showing ambient is -22 and its actually -9, this could be the fault, where the freezer is getting mixed messages and unable to regulate the internal temperature correctly.
Failing that it could be the compressor and cooling system is starting to fail (but unlikely) its worth checking there is sufficient ventilation at the rear and sides, so its not overheating.
In terms of troubleshooting and resolution, I would start by checking the door seal is making good contact all around. Secondly I would check the rear and side of the freezer has sufficient room for air to circulate, and if they all come back as ok, I would look to get someone out to strip down the unit and test the thermostat and control board.
If you would like me to recomend someone to take a look at the freezer in your area, please provide you postcode.
The control board would be a rather expensive part to replace, your looking around £125 in parts (which can be sourced here), the thermostat would be around £50 (and can be sourced here), its about 30 minutes labour to swap out either.
From what you have said, I think its most likely to be a faulty thermostat, and the engineer should be able to come to site verify the faultily component, order the part, and then return and replace it all for £35-50 in labour costs.
I wouldnt recommend you swap the thermostat yourself, as it can often be a challenge to strip the freezer down to get to the thermostat cables without damaging the plastic fascia. I would look to bring in an engineer to give you a quote, and then decide from there if you want to replace the part, at least that way you also know for sure the thermostat will fix the fault.
I would recommend you use Oakley domestics, they can be contacted on 01353 760063 or 07823 533789, we have referred numerous people in your area to them, and have always had a good good feedback.