Barry said the other day that he wished he could find Fissidens celticus. I think the best plan of action is to look in a steep-sided, shaded stream with clayey banks, and look for areas that slump off every few years. Yesterday's colony, near Gelli Aur, was one of the most extensive I've seen: F. celticus dominates the banks above the stream. Here are a couple of pics to give a search image. Of course it's possible to find small, scattered colonies of this species in various places, but sometimes it can be really obvious! Fissidens curnovii was also present in quantity.
PS look for the narrowness of the shoots in the field, don't get hung up about nerve kinks!