I was at Merthyr Mawr today for a site meeting with Duncan Ludlow, and having mentioned Habrodon perpusillus while we were chatting in the car park Duncan offered to show me the sycamore which hosts this rare moss. Despite Duncan leading me to the exact multi-trunked tree I couldn't spot it, but I nipped back afterwards for another look and found numerous small patches on one of the trunks (circled yellow in photo below).
I was struck by the superficial similarity to young Cryphaea, as Sam and Barry have commented previously.
The good news is that Duncan has removed the Holm Oak that was shading the host tree, and plans to fell further Holm Oaks slightly further away. Hopefully this will allow enough light onto the sycamore trunk for the Habrodon to persist.