How can I fix my Hotpoint Mistral Fridge Freezer?

The freezer temperature has started to drop over the last few weeks - today it reached -16 C and I have to turn on the FF to get it cold again.
The fan is running and the condenser is hot. One feed pipe to/from the condenser is hot - too hot to squeeze and the other is cool, suggesting that the system is running but not keeping the freezer compartment cold enough.
The fridge compartment stays at +3 C and appears stable.
The condenser is a Matsushita D91C18RAW5.
Are there any tests that I can make to determine the cause of the problem? Any suggestions very welcome.