Hi Mick
Does the horn work ? If not, it's the fuse that has blown. Horn and brakes share the same fuse, and if you have fitted a new/larger horn it may well have exceeded the fuse.
Or it could be the switch on top of the brake pedal - pop the connector off and check for voltage there. If 12v is present, then bridge the switch with a test wire - if the brake lamps come on, then the problem is that you need a new switch. If the lamps are still failed, you'll have to trace the wires

It's a small world after all. It's a small, small world