You said somewhere that Sybil and Cyril were baptised together as Lipscomb in 1905 have you seen this record ?
We don't know yet it Ethel made up the name Ambrose or actually married someone with that surname .
I like Isa s finding of George Ambrose Huntly. .he could be Sybil birth father but u likely to be Cyril s father if he married in Bedford in 1902
The Baptism was in Hammersmith 1905, The parents were noted as George and Ethel Lipscombe, they were living at 27 Turnville Road, Hammersmith. After doing some digging I have found that this is where a Kenneth Mackenzie lived, who i believe to be Ethel's Uncle (Ethel's Mothers Brother).
I found Kenneth Mackenzie (and some other Mackenzies) living at this address using the electoral registers and a few censuses, however there is at no point, a mention of George Ansell or Ethel Lipscombe (although there is at one point an Arthur Lipscombe as "Boarder"), so I am assuming that they were just visiting (perhaps George was working in the area).
I think my sister said that George Ansell may have been married to an Alice Gardiner, after a split with Ethel, this would make sense as to why Ethel ended up living with Cyril in Poole.
I was going to have a look to see if I can find George Ansell and Ethel B Lipscombe anywhere else together (probably in london still), But I feel that this isn't going to help me find Sybils Birth records.