Avening and Hankerton are relatively close to each other - try Google maps
http://maps.google.co.uk/maps?hl=en&ie=UTF-8&rlz=1T4HPEA_en-GBGB236GB236&tab=wlAs I've already stated Hankerton parish records for this period are indexed on the BVRI - for information on the BVRI (a set of CDs which add further records to the IGI) see
http://genoot.com/downloads/BVRI2/SELBY, William
Christening Date:16 Feb 1812 Hankerton, Wiltshire
Father:James SELBY
Mother:Ann
SELBY, Mary-Ann
Christening Date:8 Jan 1815 Hankerton, Wiltshire
Father:James SELBY
Mother:Ann
SELBY, Thomas
Christening Date:3 Mar 1816 Hankerton, Wiltshire
Father:James SELBY
Mother:Ann
and as already given Rebecca
SELBY, Rebecca
Christening Date:21 Dec 1817 Hankerton, Wiltshire
Father:James SELBY
Mother:Ann
Poole Keynes Wiltshire where a James and Ann Selby have been found on the 1841 and 1851 censuses with younger children born in the same place does not have any coverage either on the IGI or the BVRI. This website allows you to see what parishes do have coverage on the IGI (both Anglican and non-conformist) and for what periods and allows you to access just the index(es) on the IGI for individual parish registers.
http://freepages.genealogy.rootsweb.com/~hughwallis/SELBY, James
Christening Date:15 Apr 1787 Hankerton, Wiltshire
Father:Thomas SELBY
Mother:Mary
Poole Keynes was in Cirencester registration district
Possible death registrations for James
Deaths Dec 1852
SELBY James Cirencester 6a 216
or
Deaths Mar 1854
Selby James Cirencester 6a 226
1861 census RG9 1783 folio 97
Poole Keynes, Wiltshire
Anne Selby 71 Henton, Wiltshire, Servant Widow Housekeeper
1871 census RG10 2646 folio 26
Somerford Keynes Wiltshire
Ann Selby 81 Head Widow Formerly Ag lab's wife Trowbridge Wiltshire
living next door to her married daughter Eliza Radway and her family
Deaths Mar 1874
SELBY Ann 85 Cirencester 6a 26[_9]
Not sure where that gets you with Ann's birthplace. The marriage of Ann and James Selby is not showing on either the IGI or BVRI.
Regards
Valda