Think I got him!!! The list of Labour MPs on Wikipedia is very defective, there being several who I have known personally being absent; plus one who luckily is absent! (Oswald Mosley unfortunately). However; once I went through the list unfortunately the name started with W, so it took some time. But it has to be Sidney Webb, MP Seaham Harbour 1922-1929; 1st Baron Passfield (created 1929), joined the Fabian Society in 1884 shortly after its foundation. If not him, then there are at least 30 others listed in the Wiki who were North East Labour MPs in the 1920s and earlier. Whether or not they were also Fabian Society members is to be determined; but I would be very surprised if NOT Sidney Webb. Much about him on Wikipedia, also try Fabian Society Website.