Right guys one of my MOT failures was lack of horn

seeing as the current steering wheel boss has no horn button, this could be why

i've bought a new boss kit with horn button, and was wondering... is there only
one feed to the horn button? i.e. through the tab on the stalks to the back of the boss? obviously i have no original wheel to check.
if theres more than one feed, then im missing a wire somewhere!