Yesterday evening, whilst photographing the sunset from the artificial promontory of Salthouse Point slipway (SS52349582), I came across a colony of what appears to be Syntrichia montana. var. calva. Scattered tufts were locally frequent amongst patchy, moss-rich vegetation growing on Limestone gravel and I estimated the colony extended over an area approximately 10m x 10m, though no other areas were looked at. From what I could make out in the gloom, S. ruralis var. ruraliformis was more abundant (or possibly just more conspicuous) in the community and there looked to be a little S. r. var. ruralis for good measure. Confusingly, there were also a few shoots of intermediate types, so I'm not quite sure exactly what is going on out there! I'll try and get out in better light at some point to try and sort out the Syntrichia taxa at this interesting little site.
There is one other record of Syntrichia montana. var. calva in Wales, a short account of which can be viewed HERE