I would say horn is easier than the relay - see below!!
Just follow the instructions in your user manual for changing a headlight bulb. 4 screws to remove the decorative radiator grille, and two 10mm bolts to remove the Nearside Headlamp cluster. Horn is just underneath the light cluster. The same 10mm spanner/socket will undo the horn bracket easily!!
I can't understand why everybody scrapes their knuckles doing the horn from underneath :bang::bang::bang:.
You have to take out the headlamp cluster out to replace a bulb anyway, you don't mess up any adjustment to the headlamps either it makes it a 10 minute job tops!
The hardest bit is the 4 screws maybe a bit stuck in the top of the decorative radiator grille, please don't use a power driver, just some WD-40 to release them! You need quite a large stout Phillips driver for them!!
I will post up an HOW2 sometime showing how easy it is!
I am surprised your horn was repairable. The most common horns marketed in the UK by RING are made by FIAMM in Italy, who make the original horn! The diagphrams go on them and for £10-12, I wouldn't bother repairing TBH!
I'm not sure where the relay is??? But I would suspect a horn, given the fact I'm on Number 4!!!! at 147,000 miles!