I have a sneaky feeling - when you try and put all this toegtehr in your mind - that whilst we are assuming that Ethel was a youngster, that she actually might have been older (and therefore more likely to be with William Starns senior...or the pinkertons). This would also tie in with her being the Ethel born in 1878 with Jarvis as the dad...She would be 40. Still possible to have a baby at 40.
Jervis' daughter Ethel married Edward FREWEN in 1905. In 1939 she has the correct day of birth but has given herself two extra years.
Debra 
Interesting...I do still find it hard to believe that the Ethel on Desmond's Birth certificate is not connected to a Jervis/Jarvis of some description though...somehow at some point..and that's why Desmond called him his dad when he got married the second time. You dont just make up a name like Jervis.
But then again maybe me thinking that Jarvis being Ethel's dad might be a red herring altogether...maybe she did marry a man called Jervis - somebody later on after Desmond was born and that's why he assumed he was his dad??
From family hearsay and some records to back it up, I believe Desmond spent a large part of his childhood in orphanages. However, the records which I found in orphanages began when he was 14. I will see if I can dig them out. Apparently the 'only' memory Desmond had of his mother was her putting him on a bus to Mill Hill to an orphanage...
I also have reason to believe that perhaps Desmond himself went by the name of Jervis sometimes as well or that the names Desmond and Jarvis were interchangeable. I remember my dad saying something about while he was growing up there was 'kitbag' with the initials JD on it...he was told this was his dad's Desmond's bag - it was very significant to dad because it was the only thing of his he ever saw. it must have been something handed down as my dad never met his dad Desmond.
It's the trying to find out who my great grandma Ethel Salter is and where she comes from and goes to which is the problem - I remember this from ten years ago and this is why I gave up!
It's 100years since Desmond was born tomorrow and I WILL crack this mystery!
In case you haven't read this on another post - I do also have a hunch that Desmond's dad was not British...
Thanks for all your help.