What is the a2 error code on a hotpoint fridge / freezer?

SOURCE: Hotpoint Freezer

Get a hold of CAMCO, they are the authorized parts/repair depot for Hotpoint

SOURCE: hotpoint fridge freezer icing up freezer and fridge does not work

Does your model have independent thermostats for the fridge & freezer???

If not, could be that you will need to change the Thermostat.
These are not expensive - around £10 if you buy on the Internet.
Easy to fit yourself...just turn off the power, remove the power plug from the wall, then unscrew the 'stat unit from inside near the top of the fridge (some are attached to the interior light) and remove the screw holding the thermostat tube to the rear wall.
Often the thermostat gets a small hole somewhere in the long tube, which then lets all the gas escape, making the stat useless.
You can see these small holes in the pipe if you use a magnifying eyeglass, like the kind Jewellers use.