Scooterotica

Your orange feed goes direct to the horn with the white going up via the bulb holder to the horn switch, this is a direct short to ground to make the circuit. try removing the white at the headset end and holding it to ground the horn should work. that will rule out any switch problems. I would swap the horn sounds like its soaking up all the juice you have

Thanks guys. I've decided to get a small (4Ah) battery which should fit below the under-seat storage box. I'll get a DC horn, and run it along with the brake light and speedo/parking light from the battery. That'll leave just the headlamp and LED tail-lamp to run from the AC supply.