I just bagged this species (new lifer for me) here on the Llyn Peninsular this week and its probably around now in Calderdale as well.You need to look on Hartstongue fern (Phyllitis scolopendrium ) on the sporangia under the leaves. It makes distinctive cases from the old sporangia and its possible to see the larva feeding head first in the mine it makes. You need to look on the older leaves that have gone brown. Its just a case of truning over leaves until you spot it. If you find it the larva has a black head and anal plate. If the head plate is light brown its a related species called 200 Psychoides filicivora
I seem to remember that there is a large area of this fern on the path up from the main road to Underbank near Jumble Hole clough. Its the track Sue and I took you on last year Andy. It will be a good record as I never really looked for it in Calderdale, so new record awaits someone.