Didymodon can be a tricky genus with a lot of quite similar species, confusing many bryologists for years. D. luridus is one of the easier species to identify, with characteristically wide-based, almost triangular leaves and a rounded or blunt-tipped apex. However forms occur with narrower leaves which can easily be confused with D. nicholsonii.
It is always worth looking at leaf sections of Didymodon spp. if you have a compound microscope. In the case of D. luridus and D. nicholsonii, the costa sections look quite different (see images and refer to Tom Blockeel’s recent article, linked below.Read the Field Guide account