Fissidens crassipes subsp. warnstorfii?

I popped out at lunchtime to finish off tetrad SO30C which I started late last year. My 2018 visit focussed on upland-edge woodland and streams, so today's targets were species of walls, tracks and trees. The canal bridge at Goetre (SO314057) seemed as good a place as any, and produced 32 additions to the tetrad. Highlight was what I thought was Fissidens fontanus submerged in the canal - a single 2x2cm tuft of long, narrow shoots about 2cm below the water surface.  However, I could see a leaf border through the hand lens and I didn't think F. fontanus was bordered, so I collected 3 shoots for checking.

 

Sure enough, Fissidens fontanus is unbordered, and working my plant through the key in Smith took me to F. crassipes.  The canal plant was unlike any F. crassipes I had seen before, and I am very familiar with the short (<1cm long) plants of this common lowland moss.  Just in case, I checked the text in the European guide of Frahm & Frey (ed Blockeel), and there found three infraspecific taxa of F. crassipes.  One of these - var. philibertii - matched the canal plant very well, with shoots >2cm long (vs up to 1cm in var. crassipes), and with the upper part of the lamina shorter than the sheathing part.

 

the two blue bars are the same length, showing the upper part of the leaf
is shorter than the sheathing part

Frahm & Frey mention four synonyms of var. philibertii: var. submarginatus, Fissidens warnstorfii, F. mouretii and F. sublineaefolius.  This showed that several people have considered this taxon to be sufficiently different to typical F. crassipes that it deserved species rank.  A quick check of Tom Blockeel's account in the Atlas indicated that this distinctiveness is currently recognised at the subspecies level, with World Fissidens expert Maria Bruggeman-Nannenga calling it subsp. warnstorfii.  The Atlas describes this as a Mediterranean plant, and I have not yet managed to find any reference to it being recorded from Britain before.  I doubt that many of the boats that ply the canal have come here from the Mediterranean, but introduction via aquatic plants seems plausible.  Alternatively, the Monmouthshire plants might be a morph of subsp. crassipes that grew very tall under the shade of the canal bridge in last summer's drought, but if that is the case it seems surprising that it would so closely resemble the Mediterranean subspecies of this moss.