Hotpoint Iced Diamond RF64 – Freezer not freezing

I’d be grateful for any help/advice with diagnosing/remedying the problem below.

I manually defrosted the freezer this weekend (I have done this a number of times before without any problem). After putting the food back into the freezer and switching it back on, the red temperature warning light is now continually lit (the green power light is lit, as normal). Both the fridge and freezer are working, but the freezer is not cold enough to freeze food: it is colder than the fridge, but is not much below zero degrees Celsius. The fridge appears to be working normally. The condenser starts and stays on long enough to maintain the correct fridge temperature, but it is not staying on to drop the freezer temperature to its normal operating level.