This Dianthus had been flowering very well for months, then a patch died-off, then another - so now there is just a small bit remaining. The patches seem fine, then turn straw-coloured suddenly. There are cats around, but adjacent plants (including similar Dianthus) are not affected.
I have not dug it up yet, but that will happen soon - so I may have a useful update to add (eg. to say it was vine weevil).
Meanwhile, any ideas?