I located the specimen in my collection and it became immediately obvious, on getting it under the scope, that it was nothing like Dicranella. In fact it is a very skinny Philonotis with shoots just a little over 1cm tall. I struggle to comprehend how I came to the conclusion that it was D. rufescens - I guess the red stem, small size and habitat were enough to convince me.
I'm not entirely sure which Philonotis it is. The shoots are somewhat falcate-secund which, coupled with their skinniness, suggests it could be P. caespitosa. I've checked the older leaves microscopically and none seem to be pleated, or with recurved margins, which fits with caespitosa rather than weedy fontana - but I'm not sure the cells are big enough. The photos below might help - if not I'd probably best pass the specimen on to Sam.
