They do look different from other Marsh orchids; the leaves were narrow, around 8mm wide, mostly faintly spotted and keeled. The flower spike of 29 of the 44 I counted was distinctly one sided, just like the Narrow-leaved marsh orchids I examined on Skye. The labellum also looked different, some way between that of a Common spotted and Southern marsh. All were a pale pinkish purple, far less striking than those on Skye.
I am not sure if the taxonomy is fixed. Skye also has narrow leaved orchids; they used to named Lapland Marsh Orchids (a brilliant name reminding us how far north Skye is, as with other plants found there such as Iceland Purslane), Dactylorhiza lapponica. These are examples of plants I found there last year in late June.
Molecular comparisons with European populations reclassified these as Dactylorhiza traunsteinerioides along with other scattered northern populations in Britain in, for example, North Wales and North Yorkshire (the plant is never common). By contrast it has been suggested that the southern populations, those south of a line from the Severn to the Wash, be regarded as a variety (?) of southern marsh orchids, D. praetermissa. Whether this is now the accepted position, I am not sure; I am not at all sure in fact how taxonomic changes are communicated to the wider world (probably not well, judging from my colleagues - all very competent botanists - over this one example).
So whatever the current taxonomic thinking thankfully the vernacular name works rather well for the plants at both locations.
Inevitably with Marsh Orchids, one plant we found today defied classification, and might well be a hybrid between Common Spotted and Narrow-leaved, with heavier spotting on the leaves consistent with Common spotted, which were coming into flower nearby. It is probably not worth dwelling over however.