Q: I'm now busy trying to unravel the fault preventing the headlight low beam dip switch or relay from operating.Any ideas?

A: Tracing electrical problems is difficult and as far as S1 cars are concerned my experience is quite limited. It is not clear to me if the dipped lights never come on or whether there is an intermittent fault. I imply that you are happy with the relay, fuse, wiring, etc. and that if you run a current supply directly to the bulbs they turn on. A very common cause of these problems is bad earthing which on S2 and S3 cars is via the ring nut which fixes the whole light switch complex to the steering column. I do not know if your S1 has a different earth pathway but it is worth checking it out.
Alternatively, there might be a poor connection (dirt, corrosion, etc) somewhere and the only approach is a sytematic test of all connections. Fuse holders are known to get covered with a high resistance layer of oxidation. If the lights can occasionally come on, almost certainly you have a bad earth problem somewhere. Good luck, Andrea