Thanks for the response, Luggsey. However, I have just realised that I may have misled you (and anyone else) in my original post. The fridge actually gets down to around PLUS 3.5 degrees as it should do. (Not the -3.5 deg I put in the post!) I am also pretty certain that the compressor is not overheating since, although it gets quite warm, its own thermal switch is not tripping. The real frustration continues to be that I have ne idea how to access the other components. There are no visible means of access behind the freezer compartment and the unit construction uses the two side panels to overlap the fridge and freezer compartments at the front and to which they appear to be stuck (as in glued). Oh for the days when you could get service manuals!
In the absence of advice to the contrary from anyone else who has managed to service one of these units eventually I shall just have to bite the bullet and strip it down by unsticking everything.