Ciarán Byrne @kiwibyrne
Anyway, having dissected it under the stereomicroscope and looked at the cellular structure of the leaf under the compound microscope, I could have identified it without either if I'd just followed the key.
It is Pointed Spear-moss, Calliergonella cuspidata. I've nicked the photo from Ciaran Byrne's twitter feed. Ciaran is an amazing ecologist and prolific tweeter. I would urge anyone with Twitter access to follow him.