Hi Colin,
Using the Wilts FHS CD of marriages I see that William Durham was aged 21 when he married Sarah Eatwell in 1798. As a Widower this marriage was by licence dated 26-6-1798 and they married the same day in Highworth. Bondsman being; John Darby, Highworth.
There are some transcripts for South Marston on-line and I found the following which may be useful;
Bapts in Sth Marston;
19-1-1769 William Derham s/o Thomas & Mary
18-11-1770 Hannah Derham d/o " "
12-9-1773 William Denham s/o " "
18-7-1779 Thomas Derham " " "
Using the same transcripts I found these burials;
28-9-1773 Thomas Derham
30-6-1788 Mary Denham
17-6-1805 Thomas Durham aged 74 years
As there appears to be various spellings of the surname, I have found what may be his first marriage in the Wilts Marriages CD;
22-101796 Highworth;
William Derham, otp to Jane Kilmister, otp
Just need a burial to confirm its the right couple.
Alan