I would think it is either:
Jan/Feb/Mar 1854
Gateshead ref 10a 435
Sarah Cain
Or
Apr/May/Jun 1855
Gateshead ref 10a 474
Sarah Cain
OR, if born before the "marriage", then there are Sarah Halls born in 1853, 1854 or 1856 in Gatesead Reg district.
It is not made easy by the fact they didn't seem to actually marry. So we don't know if the big gap between Sarah and the next sibling indicates they were from separate relationships, or if Sarah senior and william were together but just didn't have a child in that time.