Thanks for looking. Cooper is always a possible alternative.
According to the 1851, 1871 and 1881 censuses she was born 1813-1814. The 1861 census would have her at 1822, however. I've always assumed the later date is an error. The 1841 census has a Sarah Cowper born approx 1811 in Banbury, Oxfordshire, so that fits with the earlier date, and her second marriage was in 1845, so that too fits the early date hypothesis.
But I've also noticed a Sarah Cowper born approx 1826 in the parish of Aston Le Walls, Northamptonshire, but still in the registration district of Banbury. That's a very long shot given everything I've said, but I might have to try it if we don't come up with anything in Banbury proper.
FreeBMD has the marriage of a Sarah Anne Cowper in Banbury in 1847, but she can't be mine. So it does look as there might be two Sarah Cowpers born around that time. Funny we can't find either of them!