Speedo stopped working!?!?



My speedo has been playing-up & I was looking for some help with diagnosing the problem. :?

To give the full picture; I first noticed a problem when I pressed the horn and the speedo went up. Next it developed when I pressed the horn and the speedo stopped working. A few days later when I took the car for a quick spin it started up again. Now it seems to have packed-up all together!!! Any ideas???? :?

My first thoughts are earthing and speed sensor, but the intermittent operation makes me doubt the speed sensor. The only constant problem has been the horn affecting it.

Any ideas or guidance welcome. Thank you in advance!

Cheers! :)