Hello rich73,
There is a marriage on Freereg as follows:
25 December 1832, Shepton Mallet, St Peter and St Paul. William Williams, bachelor, of this parish, to Hannah Brabner, spinster, of this parish.
The witnesses were: Ann Brabner and David Gaite. The notes state, "Banns with the consent of parents".
This last comment indicates that one, or both of them were minors at the time of marriage. From the years of birth you suggest, Hannah would have been about 20 when she married.
What information do you have that suggests William was born in Shepton Mallet in c1803? There are a few baptisms for a William Williams in Shepton Mallet but none in that year.
http://www.freereg.org.uk/The details of Hannah's baptism, plus those of her siblings are on Freereg also.
Jo.