Thanks so much for your response Pizzaguy.

There is no power to either the fuse or the relay. Whilst digging around under the footwell I found a damaged and loose ground (what we would call earth) cable. I assume that the only ground cables leaving the footwell would be for the horn and for the brake signals. Since the horn works fine I think this might be the source of my problems. Is my assumption correct?

I cannot access behind this cluster of cable at all and so I thought I could route a cable direct from the relay back to a join with this loose wire. Am I right in saying that the ground from the brake switch (and hence this cable) goes straight into the relay at pin 85?