So....it seems we have Marriage, Emma Jane Mann, age 17, (1843) father Milburn, as birth cert.
Possibility...only spec....Jane Barnfield, raised by grandparents, had no way of bringing up a child at young age, Milburn (father) & family took the child as theirs, and was Emma 'JANE' Mann

Or there is some connection between Barnfield & Plaster that cannot be seen as yet

IGI extract record
JANE PLAISTER Christening: 13 MAY 1845 Tetbury, Gloucester Mother: JANE PLAISTER
This could possibly be another Jane Plaster that does not belong in your family line

or is she, as jorose pointed out a possible dau of Milburn?
This does not help Rupert

Emma Jane...or Jane, (maybe the same person), mother not confirmed but father is Milburn.
Jorose...Tati.... jump in at any time.....
Cas