Hi Angus, If you have a part number for a replacement door seal, then one would assume it can be replaced.
I know of an older version zanussi where the old seal is cut away with a craft knife to reveal a channel into which the new seal is inserted.
Before doing this confirm it with a zanussi agent.
Also try washing out the driptray located on top of the compressor as this gets hot to evaporate the defrost water and any milk in it will turn sour and smell.
big tone the fridge man
February 2009