Bosch KGH34X43GB/09 Fridge Freezer not cold

I’m trying to diagnose why the freezer is not freezing. The fridge seems about the right temperature. The freezer is about fridge temperature and food is not freezing. The problem has developed gradually over a few weeks.
The compressor is running and is hot to touch, but the bottom 3/4 of the condenser coils are cold and are only slightly warm at the top so not much heat is being shifted out. Coils are dust-free.
I removed the rear panel inside freezer and took out a lump of ice but this hasn’t made any difference. I think this area is air ducting between fridge and freezer. The evaporator fan is turning freely and I can hear it running when I open the door.
There is no ice on the evaporator coils and the defrost is working. (Different story but I know this because the pipe taking the melted ice out to evaporate from tray over compressor broke loose years ago and dumps the melt water on kitchen floor… repeated epoxy and gaffa tape repairs but it always comes loose again eventually…).

What are the most likely causes, if coolant loss is unlikely? Anything worth checking with a multimeter before I call out a technician? If it’s the compressor, is this worth repairing?

Thanks for any useful advice.