Valve replaced under warranty. Interestingly, when I went to have the valve replaced the garage advised that they since I originally reported the fault they had replaced valves on another three cars where the cause had been corrosion. A cheap fix to the problem presumably would be to fix a full-length dust cap covering all of the valve stem. I don't know if such a thing exists however as I've not been able to find anything l online.
Cheers
Twempie