Is that "off" the Ford KA ?
If so - don't bother - the horn itself is a very simple piece of mechanics and rarely goes wrong - especially on a newish car like a KA. It will more than likely be a fuse or a bad connection on the horn press or a wire come loose actually on the horn itself etc. Check the horn on the car - Locate it by sounding it and follow your nose - it's usually on the lower part in the engine bay. Have a look at the one that works (obviously) and compare it to the one that doesn't - see if anything is obvious. Check the fuse too - it may have the sign of a trumpet next to it.
If you locate the duff horn - it's usually just bolted on - take it off and test if using a spare battery and a couple of wires - or a test meter. If after you've looked at evrything else it is duff (you shouldn't really sell a car with a duff horn - it wouldn't pass am MOT and would be classed as unroadworthy) replace it - it would cost you a couple of quid at most from a scrappers.
Hope this helps.