So the Magnum beastly loud twin horns are on the bike. I fitted them either side of the pulse air valve (whatever that is, but that's what it's called in the owner's manual diagram). They fit easily using the supplied mounting brackets, there are four of them so I used two on each horn for double strength. I fitted them between the PAV and the frame rather than on the front of the PAV, this saves grinding the mounting brackets to fit. They extend out each side just a couple of inches and clear the forks on full lock by half an inch.
Once I'd deciphered the wiring diagram, which is printed on the packaging too small for human eyes to see, the wiring was simple. I had to take the rear bolt out of the fuel tank which enabled me to hinge it up enough to thread the wires back to the battery, and to cable tie them to the existing loom. I untied the two original horn wires from the frame and folded them back under the tank, from where I extended them to reach the relay, which I attached to the RH battery housing screw. Just one new wire then had to be run back to here from the horns. I can detail the actual connections if anyone is interested and wants to avoid the wiring diagram.
Everything works fine, and they are LOUD.
Below are a couple of not very good picture but perhaps it will give an idea of how the horns look mounted on the bike. I have removed the original horn since taking the pics.