[Merry]

Jane didn't marry John (at least I can't find a marriage, can you?)

In 1851 we have

John Jago 46
Jane Blake visitor 35
Richard Curtis 19
Joseph Blake 16
Isaac Jago son 13
Edwin Jago son 8
Caroline Jago dau 2
Albert Jago son 9m

All the Jago children above are b Fowey and the older two in Golant

The baptisms for the children show:

Richard Curtis Blake bap 29 Jan 1832 Golant, mother Jane Blake
Joseph Kendall Blake bap 12 Jul 1835 Golant, mother Jane Blake
Isaac Blake bap 25 Jul 1841 Fowey, mother Jane Blake
Edwin Blake bap 27 Sep 1843 Fowey, mother Jane Blake
(Caroline is named Angelina on the baptism, by the look of it...)
Angelina Blake bap 18 Feb 1849 Fowey, mother Jane Blake
Albert John Blake bap 23 Feb 1851 Fowey, mother Jane Blake

None of these records above give a father's name, but when Caroline and Albert were registered for birth the surname Jago was used and mmn Blake:

JAGO, CAROLINE ANN mmn BLAKE
GRO Reference: 1848 J Quarter in ST AUSTELL Volume 09 Page 19

JAGO, - mmn BLAKE
GRO Reference: 1850 S Quarter in ST AUSTELL Volume 09 Page 13

So, I'm pretty sure John Jago will be named on these certificates as the father of his step-daughter's children.

Note that these two children are the only ones born after the death of Fanny Jago.

Deaths Sep 1847
JAGO Fanny St.Austell 9 7

So, what about Isaac and Edwin? The vicar says their mother was Jane Blake, but also John Jago says he is their father on the 1851 census, eve though his wife, Fanny, was living when they were born!