Derek Funcks
Senior Member & Supporter
Thanks for all the suggestions guys, although several of them I wouldn't put into the "REALLY cheap" bracket!
In the end I plumped for some no-name Chinese ones from the 'Bay at £3.79 each as a starter, given that they won't get very regular use. They're broadly similar to the ones Iain suggested above, although not the same. You can change the volume, tone and sensitivity, and they claim to be water proof.
So we shall see how we go, but if these give up the ghost fairly quickly I know I'll need to spend a bit more.
Old Chinese proverb " Buy Cheap, Buy Twice "