I was looking for stories of Eagles carrying away children for another thread and dug out Strange Victoriana (Tale of the Curious, the Weird and the Uncanny from our Victorian ancestors) by Jan Bondeson (2016). The book devoted nine pages to the Ghost of Berkeley Square, sensationalised in Mayfair Magazine in 1879. A quick skim through: housemaid staying at no.50 went mad with fright, a gentleman who didn't believe in ghosts stayed in same room and found dead etc. Notes & Queries, a weekly magazine devoted to antiquarian pursuits, picked up the story and claimed the house's haunted reputation was already established by 1872. Elliott O'Donnell, celebrated ghost hunter, then wrote a book in 1923 Ghosts Helpful and Harmful where he introduced the story of two cockney sailors Bert and Charlie entering the house and getting a scare. Bert shinned down the drain-pipe, Charlie was found the next wandering around Berkeley Square in a state of insanity (it was Bert who related the tale). In O'Donnells book Ghosts of London (1933), the sailors Bert and Charlie were now Bill and Mick - Mick jumped headlong out of the window after encountering the ghastly horror and broke his neck whilst Bill ran away and found a policeman. This changed again in O'Donnell's re-telling of the story in Phantoms of the Night (1956).
In 1907, the ghost hunter Charles Harper introduced the idea that the secret of the house was a deranged lunatic shut up in one of the rooms by his brother, a Major Du Pre. In 1928 a correspondent to the Daily Mirror helpfully corroborated the story and Harry Price, ghost hunter,
discussed the Electric Horror of Berkeley Square at length and offered to investigate any troublesome poltergeists for Maggs Bros, the Antiquarian booksellers on the site from 1938-2015. Maggs Bros reported no ghostly activity during their tenure - in fact no resident had ever reported any kerfuffle during their period of occupancy. The list of additions to the story of the Nameless Horror covered another 4 pages of Jan Bondeson 's book. Unbelievable though it may sound, contributions from the internet really didn't clear the waters.
As to the origins of the story; according to an account in Notes & Queries, an anthology of spooky tales called Twilight Stories by Lady Rhoda Broughton had been published. One story 'The Truth , the Whole Truth and Nothing but the Truth' published in Temple Bar magazine 1868 was virtually identical to the Berkeley Square ghost story. When asked if she had based her story on the Berkeley Square haunting she replied that she had in fact heard it from informants in the country. I'm sure the legend won't go away just yet.