Debbie,
Thanks for trying! I've found 'my' Sarah Cowper on the 1851 through 1881 census (by which time she is Sarah Mabbs). You can
see all the details here, if you like.
Based on those censuses, it's possible that in 1841 she might be one of:
Sarah Tee (b. approx 1816), at Church Street Rectory House, Hackney
Sarah Cowper (b. approx 1811 in Oxfordshire) at Wardington, Lower End, Cropredy, Wardington, Oxfordshire (ruled out by your 1851 find)
Sarah Cowper (b. approx 1811) at Ludgate Street, St Martin Ludgate, London
So here's my theory: The family you've found of Elizabeth Cowper is the right family, but the 1805 Sarah died, and Richard and Mary had another Sarah in another parish sometime between 1808 and 1816. There is some evidence for this: Ancestry lists in its Banbury parish records the death of Sarah Cooper, daughter of Richard, butcher on 26 October 1809 (unfortunately it doesn't list baptisms for the same period).
So thanks for your work - at least I have a reasonable theory to test in the records.