There is an interesting article in The London Gazette, in September 1976, about someone who changed his name by Deed Poll.
I can't show his name, because he is still living, but possibly the son who has a tree on Ancestry.
Several other trees on Ancestry, none of whom has found a marriage for Ellen.
Ellen appears on a couple of Electoral Rolls in Barnet in the 1960s, living with her mother, but no husband.
John is shown as John Bell Borthwick on two of the trees, born in Berwickshire or Co Durham in 1911.
May not be the same person, but there was a John Lesie Borthwick, mmn Bell, born in Lanchester District, but in 1905. He is on the 1911 and 1921 censuses with his parents, David and Elizabeth Ann, who married in Lanchester District in 1901.
He died in 1985 in Durham North District, having married Elsie Ramsay in 1933 in Lanchester District.
David