Thanks for all your tips guys. I have chosen to cover the heads of the nipples with "marine mastic" - a polyurethane adhesive/filler/sealant which sets hard enough to be sanded and painted, when smoothed over with lots of soapy water and bare fingers it leaves a tough, hard surface. I hope this will prevent the tube getting nipped. There are a couple of pictures here :-
http://www.bleazey.co.uk/M3W.html#PROPI would strongly recommend that you look closely at your tubes when having new tyres fitted. From the comments above it seems not to affect everyone. When fitting a new tyre and tube - naturally enough - the fitter will probably not inspect the old discarded one for damage. Being the tightwad that I am I wanted to re-use the old ones so inspected them closely.
