I have just bought this little gem online:
http://www.chainreactioncycles.com/M...?ModelID=20498 - and it really works! It's loud enough to be heard inside a car with the windows up - which is great for when a motorist pulls out and forgets to look over their shoulder. I've used it a few times so far and on one occasion it caught the attention of a guy who would otherwise have pulled out right in front of me, probably causing me to crash into his car.