Thanks to Seoras for that information. I guess (or perhaps 'hope' is more accurate!) there must be some records for the Home in the Bristol or Gloucestershire Records........
Mary Ann is in Bristol in 1939 as Ellis but with Boucher in brackets which suggests she gained that name after 1939. (Transcription record)
Hmmm but the original shows her as Boucher which has been crossed out and Ellis added. On her own, no-one with her.
Thanks again Girl Guide! This is the first sign of the name in anything formal - the source of the information to date was just my dad remembering her as 'Mrs Boucher' (pronounce Bough - cher) but he said that at his age he didn't even know her first name or dare to ask!
She married Walter James Ellis in 1903 and they appear in the 1911 Census as living in Penarth in 1911 with their two children. As Walter James Ellis did not die until 1955 they must have split up before the war.
She seems to have been living with her grandmother and an aunt respectively in the 1881 and 1891 censuses even though she was only aged 3 at the time of the former. I cannot find her in 1901 and then in 1911 she is back in South Wales, married and in Penarth.
It is indeed intriguing (and maybe a little bit sad as it sounds a pretty chaotic life).