Just a quick update: I ordered a resistor kit from Rezpax (£7+ incl, postage) which arrived yesterday. I soldered the new resistors in place (luckily I am of the generation used to working with soldering irons, valve-driven radios, etc) replaced the unit and the fan now works better than ever - all 4 speeds working!
I still have to replace the mesh grille that fits under the air intake panel in front of the windscreen - couldn't find anyone selling these so have ordered some fish-pond filter which ought to do the job.
I am still pondering how to open up the unit to allow me to clean out the dead leaves that must be sitting on the intake side of the evaporator unit. According to microcat I have the early version of the air-con unit, which doesn't have a filter (later ones do) and there is no obvious way of getting the debris out.