Good morning Lilian
Do you have access to all censuses ? If not I'm happy to post some details for you.
Meanwhile - here's how I got to Hankerton.
1881 census - Rachel says b. Hankerton, Wilts.
1871 census - Rachel says b. Hamerton, Wilts
1861 census - Rachel says b. Ankeston, Glos
1851 census - rachel says b. Ankeston, Glos.
I googled Ankeston, Hamerton and Hankerton and Hankerton, Wilts was the only match. Hankeston is about 5 miles as the crow flies from Avening.
However, as Valda says, there are no Selwyns living there on any census and I couldn't find any on any census who said they were born there either.
There are a few Selbys tho. Living there and born there. I wonder whether it might be an idea to get another of the birth certificates - since your Fredericks is only a partial match? Does Rachel sign or make her mark? Perhaps she couldn't read and write and the registrar wrote what he thought ?
Meanwhile - I'll keep looking any see what comes up