I thought that it might be due to some sort of nutrient deficiency but the reverse (underside) of the fronds `gave the game away` - it was heavily infested by the micro-moth Psychoides filicivora.
I`d noticed a few adults of this species on the plant and on the adjacent window in the spring, when I`d first brought the plant indoors, but obviously they have since and laid eggs on my indoor fern. Here - free from predators - they are doing well (too well!) and I`d better take the plant outdoors or Psychoides filicivora may become a troublesome house moth here!
Search your local hart`s-tongue ferns, but be aware of the related P. verhuella. You will need to check the larva`s head - one species is very dark brown (verhuella), the other (filicivora) a much lighter brown. Just `google` the species` names on `UK Moths`for good photos.