Some considerable time with Google Streetview failed to reveal a precise location. In desperation, I turn to the National Library of Scotland's very considerate Internet provision of archive OS maps. On the 1906 series, Plasbâch appears at Lat/Long (52.32367,-4.08583) (or GR SN5795271894 if you prefer), [click to enlarge]
On arrival, of Plasbâch there is no sign on the ground whatever: close by is a rather later property. Nevertheless, the box is there and at a very jaunty angle to the horizontal indeed. Someone taking note of HMHB, the hedge has been cut. I can't believe many envelopes get stuck in this fella, so catch it while you can.