Thank you both for the information. The article about Alexandra Palace is fascinating.
I will keep investigating, but they certainly seem to disappear from the records after the 1911 census. Mary Ann is the youngest of the eleven Jones siblings, most of whom are in and around the Hackney area, but she and Oscar junior do not turn up in any of their households, as far as I can tell, which makes me think they must have gone to Germany, either voluntarily at the end of the war or by being deported.
I wondered whether the other hairdressers listed in the household in 1911 - Ludvik Besta (born Bohemia, listed as Austrian) and Reinhold Kitzendorf (born Kentish Town, listed as British subject) -might offer any clues?
I also wondered if the Lutheran Church in Hackney held any records. Oscar junior was baptised there in May 1914 and his godparents are the Poehl family, who were also hairdressers who Oscar senior was living with in the 1901 census.
It seems terribly sad that Mary Ann feels 'sneered at' by her family. Although entirely anecdotal, Mary Ann's sister Eliza Thompson names two of her children Cyril and Evaline May, the same names as Mary Ann's triplets who die in 1906, which suggests that the family must have been supportive at that time.
Thanks again for your help.