It looks like Jane was born before the Marriage.
Possible Birth Reg;
JOHNSON, JANE - no mothers maiden name, so illegitimate      -  
GRO Reference: 1854  J Quarter in MORPETH  Volume 10B  Page 221
(GRO)
FreeBMD
Marriage Reg;
Sept 1856
JOHNSON    Margaret  Newcastle Tyne    10b   172
Robert SWAN on same page
Mabel is not reg until after the Marriage.
You will have to buy the Cert to find more about Margaret/Robert as M/C does not appear to be on line or in anyones Trees.
Having said that I think this is Margarets Baptism on FreeREG;
28 Sep 1834, St Mary the Virgin, Morpeth
MARGARET Johnson, Daug of WILLIAM/MARY
Abode Cottingwood
Father a Woodsman
1841 Census Morpeth;
William Johnson 30 Woodsman
Mary Johnson 30
George Johnson 9
Margaret Johnson 7******
Ann Johnson 5
William Johnson 3
1851 Census
William Johnson   40 Woodman born Morpeth
Mary Johnson   42
George Johnson   18
William Johnson   13
Jane Johnson   9
Barbara Johnson 6***
Robert Johnson   4
GRO
JOHNSON, BARBARA  mothers maiden name MARSHALL**  
GRO Reference: 1844  J Quarter in THE MORPETH UNION  Volume 25  Page 30
Marriage on FreeREG    29 May 1831 Morpeth William Johnson to Mary Marshall
Bapts on FreeREG also for SWAN
Parents ROBERT/MARGARET
Mabel 10 May 1857
Alice Singleton 19 Mar 1865
Thomas Antony    19 Jul 1868
Mary Hannah 1 Oct 1871
Cant see a baptism for John W Swan born c 1860
Trish 
