In 1908 16 York St Covent Garden was the office of TACT The Actors Children Trust. A charity who looked after children whose parents had been actors.
I see that Graham Roy Hylton Daviss was probably born just after the census of 1891 - do you have the address from that certificate, and have you been able to find the family in the 1891 census? (And later censuses, too - I'm having trouble spotting them, so it would be helpful to see if they're together/which names are used when).You might be able to trace their movements between censuses with archive.thestage.co.uk .It is possible that Alma and Graham didn't marry until later in life, or never married at all.
Mervel (transcribed on Ancestry as Meriel) had banns read as a Clavering Craig in Charlton and according to this record married a Clair May Brown of the parish of St James', Farmley, Yorks on 7 Nov 1918. I can;t find this on FreeBMD to check what name he used at the actual marriage (or much evidence of her with that spelling of Clair either)
Cate Have you followed Vera?I see she had a daughter who married and she had a son. In both cases using family Christian names.
Have they any idea about when Alma died, do they know if Vera living with her up to her marriage and where was the last address known for Alma?