Highworth, the registration district - for marriages in Dec qtr 1850 the page numbers in the GRO index run from 629 to 659 (some are indexed as Swindon)
The William Durham marriage was page 637. Seem to be three marriages in all with that reference.
(No marriages indexed pages 636, 638)
Page 639 will be a new parish, according to Family Search the marriages with that reference were at Highworth (the parish church of Highworth, unusually perhaps for 1850 they seem to be sourced from BTs)
I think that the parish church marriages in a district were indexed in some kind of order, possibly for somewhere like Highworth it would be done alphabetically by name of parish.
List of parishes in Highworth RD
http://www.ukbmd.org.uk/genuki/reg/districts/highworth.htmlSo it looks as though the marriage might have been at Hannington.