Working fine. This one -
https://www.ebay.co.uk/itm/TM803-Fri...-/122238036405

I was confused about the range of the settings for these things,
because none of them suggested that their alarm systems were capable of
being set to negative/ below zero values.

This one can display the temperature where the display is installed
(in) or that of the remote (out) wired sensor. An alarm can only be set
for the 'out' sensor, an high alarm and/or a low temperature alarm can
be set. Rear of display is fitted with magnets, so it can be stuck to
the side of a fridge or freezer.

I have it set to alarm if the temperature in the upright freezer rises
higher than 13.5C, which is located in the utility. Low alarm set to
minimum, high alarm set to 13.5C. It triggers within seconds, if the
door is opened, but restricts itself to a few bleeps every minute as a
warning - it is just audible in the kitchen. I will eventually move the
display through the wall, into the kitchen, to make it more audible.

The freezer already had a high temperature alarm, but just a light and
not easy to spot unless you went in the freezer. I could have rigged up
a simple door open alarm, but that would only trigger with the door
open - this alarm will trigger if the door is open too long, or the
freezer fails, or power is lost.