Horns now working - loud & clear!
Last evening I removed and dismantled the horn relay (closest to driver) and found it heavily oxidized inside, but after cleaning the mechanism and winding, etc. with an old toothbrush and some solvent, it looked pretty good. A light spray of WD40 before reassembly & installation, and the horns 'kind of' worked - emitting only a short 'blip' when the horn buttons were pressed.
I then proceeded to remove and dismantle both horns for inspection and cleaning. In each case, once the plastic horn acoustic part was removed, the armature and diaphragm were stuck fast to the horn housing. To separate them I applied 12V across the two terminals, and off they popped!
Again, a good solvent clean of the horn contacts and housing windings, etc, then a little more WD-40 applied before reassembly with a smear of blue 'Hylomar' compound at the gasket joint, and hey presto! They both now emit that very sweet-sounding, twin-horn blast!
All BMW's should have horns like these.
I also dismantled, cleaned and treated the other two relays, in the same manner as the horn relay, and now the fog light works, with the switch tell-tale green lamp lit up, and high beams switch on immediately - they were a little sluggish before.
All in all, a pleasing result after an hour or so's work.
I believe the unused relay position in my car may be for the low beams - as previously mentioned by HB Chris.
Thanks again for the assistance, Chris.