Hi Ottoman, thanks for the info.
I decided to take a look last week and managed to remove the front offside black plastic grill. By pulling the bottom part forward it comes out but the top part of the grill has plastic legs that are vertical. So as you pull it forward you also need to pull it down. Fairly easy so long as you take it steady.
Then you need a size 13mm spanner to undo the single bolt that holds the Horn on.
I took mine apart and the diaphragm was rusted in parts so I cleaned it all up.
Now it sounds louder than when I 1st got the car! All good and saved £100 which is what I was quoted for replacement and fitting!