Crumbs! Lots to chew on now. Firstly, that is Henrietta in 1871. It took a while for it to come to light as Ancestry had her surname as Brodock, but FindMyPast and FS got it right. I'm not sure what term should have been used to describe her, although Sister-in-law was the easiest. Mrs Biffen was actually Henrietta's half-sister, being an illegitimate daughter of Ann Andrews.
Moving on to the John who died in Pewsey RD aged 64 in 1881, I have a copy of his death certificate, and he is definitely not my man.
I had never seen any of the Everley data, as I know the village as Everleigh, although I now see that the spellings can be interchangeable. What is really curious is that John and Ann should have had Henrietta and Frederick baptised there. Both children were born in Wilton, East Grafton church was nearby, and Everleigh was several miles away. Baffling. I am not sure about the William baptised in 1871. John and Ann were still living in Wilton for the Census that year, so I would expect to find his birth registered in the Hungerford RD, just as Henrietta's and Frederick's were. That would mean he is the subject of the entry in Oct-Dec 1869, but that boy was recorded with his grandparents in 1871 as 'Daughter's Son Illegitimate', so cannot be the son of John and Ann. I suspect in fact an error in FamilySearch, because 'William the son of Ann Lovelock (Single Woman)' was baptised at East Grafton on 25 Jun 1871. Too many coincidences for any other explanation methinks.
And lastly the John who died in the Amesbury RD in 1892, aged 78. I have no idea who he actually was as he is not positively identifiable in the 1891 Census. However, I have a strong suspicion that he was the man at Shrewton at that time, born in Shrewton, and therefore not my man again.
On paper there are 7 near-candidates for my great-grandfather's death, but none of them imply a birth in 1815. He was baptised in June 1815 ( a couple of weeks before the battle of Waterloo, not that that is significant as far as I know) and although he claimed to be 34 in 1851 and 44 in 1861 he was 56 in 1871. If Ann was not fibbing in 1881 and was not a widow rather than married, although goodness knows why she would claim otherwise, then the death of a John in 1877 can also be discounted. That leaves deaths in Winchester ('82), Wantage ('88), Kingsclere ('92) and Bath ('95). Other data eliminates the Winchester and Kingsclere men, so I am left with Wantage and Bath, both of which relate to dates before 1815 for births.
I shall puzzle on!
Many thanks for the responses.