I got this:
http://www.ebay.co.uk/itm/221675979432
Like Maviczap's suggestion, but cheap and comes with the required relay. It can be mounted without splitting it but it's neater if it is. Mine isn't split, mounted on a custom bracket hanging off the side of the radiator near where the original was. The relay is mounted on the original horn bracket.
They do wear out quite quickly though. I'm on my third in about 5 years. The first was a genuine Stebel. From what I can see there's barely any difference between these and the cheap copies.