Hi everyone, just to let you all know I have just changed the horn in my 2001 Seat Leon, 20VT model. All I did was pull out the grill surrounding the passenger side fog light using some pliers/grippers which is quite hard to do but a bit of force and it will pull out. There are 4 clips holding it in place.
After removing this, there are two torq screws holding the fog light in place - remove these and stick your hand in and feel downwards and you will find the horn.
Hope this helps someone...