Ref. Note 2641666. Note this building is attached to 58 H. St [Hamilton Smith's].  V. likely to be the missing 56. Dental practice's domain name now up for sale, but Google Maps, in its short journey down The Pightle, shows a read door with a classical pediment, attached to this buidling, or the next. GM shows address as 60 The Pightle, but Royal Mail postcodes only go as high as 21. Guess this is an error by GM. and is 60 High St. You can't get exactly opposite the red door on StVw, but it clearly shows various brass plates, typical of professional outfits such as dentists, on the pilaster [those with higher-res. monitors may be able to read them, but as the images were renewed in March 2022, it's feasible the Dental Practice had already gone.
I'd bet that The Pightle was once a house of that name, whose land was recently developed [housing beyond the junction, and the High Street's corner, look 70s, 80s-ish] and the resultant street adopting the name – a common naming m.o. – so I suspect the street replaces the location of no. 56.  It may be that NMDP, as was, occupied the upstairs of #58 but used a side-door, hence an entrance in The Pightle.