However, all was not lost! Following a trip out to Harton Hollow on Friday where I searched for larvae on Hartshorn ferns (without success) I decided to look at the one plant we had in the garden and yes, there they were, feeding on the sporangia! It is called Psychoides filicivora.
This species does not exist on my European list and it is thought to have originated in the Far East, but it is now established and spreading in the UK. Don't know how it got here though.