Sic: scaled up! I've come across a couple of the permutit plug-in cartridge
types where the waterways feeding the cartridge have become blocked up
(whether through limescale or the phosphor salt getting back into the feed
and precipitating out again, I'm not sure) and one 'Combimate' type which
seems to have the same problem as it still has phosphor balls in the
chamber despite not having been touched for years. I suspect it's an
affliction of all such designs, but I wonder if there are any that don't
suffer from this problem (and if so how they achieve this).