One of my RMs has developed a distorted horn which can be heard on this link

Horn between the sound of the bell and the buzzer.

Is the horn something I could repair or will I need to replace it?

I'd be grateful for advice on this.

My bus number (if any): RMs 238, 471 & 2213. GS17

Almost certainly the resonating plate inside the horn.
Original horns were Lucas/CAV and had a large screw and a lock nut on the top.

You can adjust the pitch by very carefully loosening the nut and turning the screw by a tiny amount.
If you are not careful, the lock nut turns the screw and it can take an age to get it right.

It could be that a spiders web and a load of entwined insects are inside the sound outlet.

Most of these are riveted and very often the resonating plate has corroded or severed.

Probably much easier to replace if adjustment fails.

It's usually damp a spray of electrical contact solution and a little light graphite based lubricant
might invigorate the solenoid.