see the answers above.

Thank you— l followed up the fern smut moth suggestion across several sites, went and inspected the ferns again, and am now certain that’s what it is. You get a different point of view depending on whether you’re a gardener (Eeek! Something’s destroying my beautiful plant! Quick, slash and burn the infected leaves!) or an entomologist (Wow, just look at these clever little camouflaged burrows!).
I checked the other susceptible fern varieties, which are also setting spores at present, and they are clear.
Thank you for your speedy response, Bamboo, and the extra links, Seaburngirl.

you are welcome.
my daughter has just graduated in wildlife conservation and has a particular interest in moths. since March 31st she has recorded 91 different species just in our back garden in East yorks. My skills have as a consequence improved too. so I recognised it as a micro moth. we now have lots of extra native plants to complement the ones I grew before.

I will check my ferns and see if I have any of these ones lurking. The perk though is you will get more predators in the garden.