Ok, so not the most interesting post but it’s been a long time coming. The horn didn’t work when I got the car. I found that one of the plungers was broken and then found out I had a Grant horn hub which are notorious for issues and worse, fires!
After a few false starts and incorrect parts I now have a GT Performance hub and horn button. I also needed to drill away about 5mm of the steering column sheath as the horn would short when turning right. But now it works!