The Sherlock Holmes Society of London has published one of their handbooks relating to GLOR, and I have investigated the location myself. Unfortunately my identification relied upon a railway which at the time of GLOR had not yet been opened ( ;(), but the SHSL I believe locates “Langmere” as the “long mere” formed by two of the Broads (Hickling and ???? (haven’t got the atlas handy )) close to Yarmouth as did I. But read the handbook, which is well worth it.