Last week I came across a Dichodontium growing on bankside reinforcements (stone and mortar) in the flood zone of the River Taff at Radyr. I assume this is most likely pellucidum but wasn't entirely confident I could rule out flavescens. The leaves are around 4 times as long as wide, which according to Smith is right on the boundary between the limits of each species. No sporophytes unfortunately. Any thoughts? Sorry my photos aren't as good as Charles's!