I replaced it only four years ago with one of these:
http://www.screwfix.com/p/bottom-ent...ll-valve/52836
yet it is now starting to drip slightly.

A neighbour once told me that this hard water area plays havoc with
ball valves as even a little piece of grit or limescale can lodge in
the valve and cause it to fail. Is this true?

Can I unscrew the large blue cap (see Screwfix product picture above)
and clean out the innards? (I have an isolating valve on the water
inlet.) Or is another replacement needed? Sure, it's only a fiver or
thereabouts, but do these things need replacing every FOUR years?