Noticed a water leak by the mains stopcock which cuts off the feed to the header tank while allowing the rest of the mains system to still be used. It's in a dry (or was) cupboard. Turned out to be leaking from the stopcock itself, and at quite a rate of knots. Rather than try and fix it in situ, I decided to change it for a new one I had lying around. On removing it, the cause of the leak was obvious - no olive in the input side. So where has it gone - some kind of electrolytic action? There's no way it's been there for many years with no leaks without one being fitted in the first place...
The old stopcock was near welded onto the input pipe, although the compression nut came off easily enough. I had to use a fair bit of force to free it, but both the pipe and tap body seemed corrosion free where they met. Just no olive.