Just reading the series 1 operators manual when I came upon this
FUSE.
The single fuse is located, together with a spare, under a separate cover
alongside the control box on the scuttle. It protects the horn, windscreen
wiper, fuel tank level unit and the stop lights.
A blown fuse is indicated by the failure of all the units protected by it, and
is confirmed by examination of the fuse, which can easily be withdrawn
from the spring clips in which it fits.
How simple can it get? no horn? check the fuse