Hello Heather
Moreton isn't a parish and does not have any records of its own. It is a hamlet in the parish of Thame, Oxfordshire.
Unfortunately I do not have a transcript of the Thame parish registers so I can't check them for you (perhaps some kind person who has access to these will do a look up for you). However, I have found a possible candidate for Thomas BULL's father in the 1851 census:
Ref: HO107/1726 Folio 320 Schedule 53 - Moreton in the parish of Thame, Oxfordshire
William BULL, Head, Mar, 58, Farmer of 98 acres employing 5 labourers, born Northamptonshire
Sarah BULL, Wife, Mar, 63, born Bucks, Haddenham
The household also contains 1 female servant
I also looked up William and Sarah in the 1861 census in the hope that I might get a more specific birthplace for William, and, although a place is stated it is not very clear:
Ref: RG9/884 Folio 155 Schedule 12 - Farm, Priestend, parish of Thame, Oxfordshire
William BULL, Head, Mar, 66, Farmer of 98 acres employing 3 men and 2 boys, born Northamptonshire, Brandham?
Sarah BULL, Wife, Mar, 74, birthplace not given
Hope this helps
Orpheus