Thomas Gwillim and Susannah Haines were living and working at Lillarsbrook House, Whitchurch in 1841 census. Also living at the same property were Caroline Meek, Daniel Meek, Mary Meek and Eliza Jones. So you say there is a registration 1840 for a Margaret Meek, but she must still be alive when our Thomas & Susannah married, as she had a daughter Margaret in 1847. So we can cross that one out. Interesting to see the Meek connection. Wondering where is Presteigne in relation to Whitchurch or any villages nearby? Having copy of marriage certificate for Thomas & Susannah, showing Thomas as a Widower at that time, does confirm prior marriage.
Now to update on other details, Thomas Gwillim was son of William Gwillim b. 1776 at Harewood and Margaret Matthews. In 1841 census, his father William Gwillim was living at Michaelchurch, aged 66 years, by 1851 census, he was living with his son Thomas Gwillim and Susan (Susannah) at Monmouth.