I've swapped the loom for one working but on both the horn has a constant electric feed. Is this right or has my switchgear gone?
Get a multimeter and check for 12V before and after the horn switch. You most likely have a shorted connection due to water or dirt in the switch. Clean the switch internals, check the wires aren't shorting and use some WD-40 to push the moisture/water out. And work the button a few times too.
