The concept of Racomitrium heterostichum sensu stricto (also known as R. heterostichum sensu Frisvoll) is much narrower than it used to be, when it also included R. obtusum, which since 2020 has been considered to be a species in its own right.
Racomitrium as a genus is widely regarded as tricky and the descriptions and keys in Smith’s 2004 flora and the BBS Field Guide are now out of date and should not be relied on. The broad concept of R. heterostichum referred to in these publications is now called R. heterostichum sensu Smith (2004) to differentiate it from R. heterostichum s.str.
The good news is that Tom Ottley reviewed British and Irish Racomitrium heterostichum s.str. and R. obtusum recently and his comprehensive Field Bryology article (which can be downloaded from this page) is an essential reference for anyone attempting to identify Racomitrium species.
Great care must be taken in identification of R. heterostichum s.str. which is easily confused with forms of R. obtusum with hairpoints, or R. affine. Generally, candidate R. heterostichum s.str. is blackish or dark green, with long, whitish hairpoints which are on older leaves as well as young ones. It normally lacks the yellow-green hue of R. obtusum and under the microscope leaves can be seen to have blackish patches. If the leaves are falcate then it could be R. affine, which grows in similar places.Read the Field Guide account