Thanks very much to everybody who helped, especially those who found Sarah's twin Alice - I had no idea. I had the baptism but, wouldn't you know it, it was the last one on the page, and I never thought to click on to find Alice. In those days (~20 years ago) I didn't know the sighnificance of private baptisms, nor that it was unusual (at least in my family) to baptise as early as 9 days. Either because twins had a poor survival rate then, or to hide the bastardy shame, or both.
Yes I have Sarah's birth certificate, and not just any old cert but the one my mother found in her own mother's effects after her death, including that marriage certificate, both of which proved she was not a Manton but an illegitimate Hancox, which was quite a shock to her.
Sorry, Sarah was a servant, a cook (her known profession) in Lillington in 1901, still a Hancox, just a short walk from her future husband George Skelcey in Cubbington - there's a romantic story about this which I'll leave for now.
Now onto the DNA match theories. If Sarah was not Samuel's daughter - and WHY would he not acknowledge her? - the only other people who could contribute both Manton and Gaydon genes to Sarah were his surviving elder brothers. John 1841 had no children of his own, so perhaps he was infertile (his wife Harriet had a daughter from before the marriage). Thomas 1844 was in Kenilworth in 1881 but fathered a daughter that very same year by his wife - seems a bit cheeky. Henry 1846 and Stephen 1852 are not found after 1861 though no deaths found - Henry seems not to be the younger one found in the Stratford area on censuses.
So my attention was focused on George Manton 1850 who at least was not ridiculously older than Harriet. He is at home until 1871 but in 1881 is a Cowman in Alvechurch, Worcs, married but alone. As an alibi this is even better than Samuel's in Sambourn but an errant father-to-be can travel a long way in 9 months.
But after that he disappears. Since I last posted I have found the Coventry George Manton married to Zelpha/Zephtha in Wiltshire, so I have no marriage for him anywhere nearby, and certaiunly none to a Sarah (per those Canadian immigrants before late 1881. Thank you for your facts - I'd like some opinions.