These photos are well known. The WD machines were made by Enfield in Redditch (and other Factories in the area,) the underground Factory was engaged in other war work. After the war surplus machines were returned by the forces for refurbishment and sale as (mostly) civilian machines. A repaint to civilian colours was often part of the work, usually over the green service paint! This work was carried out at the Westwood factory and also the Greenland Mills factory in Bradford on Avon. The despatch records show refurbished machines going out as late as 1949 and I have a reliable ex-Westwood source tell me that there were still crated machines in Greenland Mills in the 60s when that building was no longer used. The painted X means a machine selected as suitable for rebuild.
Nice picture Steve! I had seen it before, but in a much lower resolution. The caption that I saved with this picture is "1945 returned ex-War Office motorcycles cross paths with V&A artefacts being dispatched back to London".
Pictures of the situation "today" can be seen on this urban explorers website.