Hi Wendy,
I know the area quite well as my grandfather was from Burbage and I visit every so often still. I'm also the OPC for Easton Royal,Milton Lilbourne and Wootton Rivers. I had a look at the marriage of William and Mary Hillier you mentioned. Not easy! Hillier's a very common name in the Pewsey Vale (Grandad Alf's mother was a Hillier). I found four marriages from 1784 to 1793 which could be possible. Two in 1793 I disregarded as unlikely as they appear to have children within a year of the marriage in their marital village. These were William Hillier and Mary Brice in Collingbourne Ducis and William and Mary Cooper in Upavon. The third pair,William Hillier and Mary Gee in 1784 in Bromham, don't appear to have children unless they move away from the wedding place. The fourth is the more probable pair. These are William Hillier and Mary Neate in Bishops Cannings on 12 Sep 1785. Both were of the parish. They appear to have a son Thomas nine months after the marriage in Bromham. It's possible that they then moved to Wilcot but cannot be proved. George Hillier,as you know,was baptised in Wilcot on 1 Aug 1791. His parents are listed as living in West Stowell. West Stowell is a hamlet that's part of Wilcot. He appears to be their only child there.
I think it likely that William Hillier,the father,was baptised in Wilcot on 5 Dec 1769,the son of John and Mary of West Stowell. They also had a son John baptised in Wilcot on 18 Jul 1771 and a daughter Mary (19 Sep 1775). Mary is the likely mother of the illegitimate girl Sarah in 1799.
So Mary Neate is the best choice so far but not necessarily the right one. I've been through all the William Hillier/Hilliard/Helliers etc married to a Mary in the surrounding area so unless William's bride was from over the nearby border with Berkshire or they married in a non-con chapel, we're stuck with her at the moment.
Sorry I can't be of more help. Good luck!
Regards,
Linda