The horn button was snapped on the light switch. As with everything else, I didn't want to replace it, I'd rather repair it.
So, I bought a cheap Indian one so that I could swap the button for my original switch. This was a bit of a gamble as I didn't know whether I could strip and rebuild one or not.
You need to remove the e-clip from the back, then the rotary switch can be lifted out and it's easy to strip after that.

The switch is held in place by a solid rivet so I had to dremmel the rivet out of the old one and same with the new one.
A problem was that the indian button is slightly bigger that the original button, so I had to open up the hole. I used a 9.5mm drill and a little filing.

The right one is the indian one and shows how much bigger the hole is.
I've never used solid rivets before, but I got some 2mm x 3mm aluminium ones and just gently peened them over. It was much easier than I feared.

Of course, it was.tempting to use the indian ebay switch but it just didn't look right, AND it's a bit of a quest now to use as much original as possible.
