Smeg UKC325XNF: Freezer alarm going off

Hello,
I have a Smeg UKC325XNF frost-free fridge freezer, bought about 4-5years ago. Recently the alarm has been going off intermittently and the temperature reading for the freezer flashing to show that it has risen above -16 degrees. I turn the alarm off and after a while the red light has been going out and all returns to normal. I am ashamed to admit that this has been happening on and off for 3+ weeks but that for various reasons I had not got round to dealing with it. However yesterday things got much worse and now the alarm is going off every three hours or so and the warning light is on much of the time.

The fridge seems fine: temperature shows 4 degrees, seems cool, butter is hard etc. The freezer does not have any frost build up and the contents don’t show any signs of having thawed (e.g. small amount of ice cream in the state it went in, not melted into a re-frozen puddle.) When I checked just now the compressor was hot and the metal panel at the back cool (although hot when I touched it yesterday). The water container at the back is full. I don’t know how much of this is useful information – please excuse my ignorance!

We have tried the simple things recommended in the manual (reducing food in the freezer to improve air flow, dusting the metal panel at the back, turning the appliance off for 10 minutes and restarting) to no avail.

I see you ask for a serial number; I have a number of 90126 0019 1 on paperwork which came with the appliance but I’m not sure if this is what you need.
Any information or advice would be very gratefully received, including where to find a trusted repairer in my area (postcode N17 6DB) if necessary. Thank you very much.