You could also check the power feed to the horn Richard if you have a multimeter. With ignition switched on measure the light green/white cable that connects to the horn (or the spare cable of same colour for second horn) with respect to ground/chassis. There should be +12 volts on this cable all the time as the horn switch is on the ground side. If no 12 volts, check fuse 9 (10 amp).
If as John suggests the horn itself has failed then good opportunity to upgrade to matching high/low tone pair (given extra wiring already present). They are as cheap as chips on eBay.
