Mick,
Z1 enterprises is the easiest to fit + all wiring is complete + extra 10-15% output.
I bought one from electrex & while a lot cheaper, it only has standard output & 3 yellow wires...no plug, so will need a bit of faffing around to get sorted.
Z1 enterprises is the easiest to fit, jus plug n' play. + a bit of sealant around the grommet (read all about that in other threads on here).
By the way, I presume them air horns you've got are musical, just wonderin' what tune they play
