Mizz,
I've attached a set of photos that illustrate the reassembly of the horn button.
I hope that they help.
The last step involving the placement of the C-shaped spring can be a bit fiddly.
It looks like you may need a bit of repair work on the steering wheel as well. You may be lucky enough to find one on for sale on the 'net.
Let us know if you need further help.
Chris