What is wrong with my freezer?

Zanussi under counter freezer, ZFT710FW

The flashing red light which indicates that the freezer is warmer than -12 C is almost permanently on, even after I press the fast freeze button (which I can see is on because it has an orange light). Ther is also a beeping noise as part of the alarm, but this does stop when the fast freeze button is pressed. The contents of the freezer are still frozen (which is good).The thermostat range goes from -16 to -24
and the manual recommends keeping it on -18 C. I tested it with a thermometer and got various readings, the coldest of which was -15 C and the warmest - 12, even after I changed the setting to -24 C. I have left it on -24 C. The current position is that it is on its coldest setting, the food inside is frozen, but the red light is always flashing. The alarm is not ringing. The temperature is considerably warmer than the warmest possible setting.

Any ideas?