If the insurance company don't ask about it, don't mention it. (This applies to any claim ever). I doubt they will bother, they just want a list of damaged parts and the price it will take to fix so they can claim it back from other persons.
I _think_ that having a working horn is compulsory and your bike isn't considered roadworthy without one, which would invalidate the insurance, and get you charged for driving an unroadworthy vehicle, driving without insurance etc.
edit to add linky
http://www.ukmot.com/bike/6-1.asp