Although I used to live in Trecastle, I have no idea where Penfay is. However, I would point out that Llywel is the parish church which serves Trecastle, and it is about a mile distant from the village itself. I know the church well - my daughter was baptised there, and my mother is buried there.
I was interested to see that the marriage was witnessed by Thomas Nicholas. Our house in Trecastle was owned at one time by Thomas Robert Nicholas, who was a skinner and glover and the house is shown as being in Tre-Escob, Trecastle. Tre-Escob is the part of the village which used to be owned by the Bishop of St David's. Now, this Thomas Nicholas died in 1794, and his son was also called Thomas Nicholas, so difficult to know which one was the witness.
Checking on my notes regarding the history of Trecastle and Llywel which I researched whilst living there, I can see a James Powell, Gentleman, of Devynnock in 1841. I'll report back if I find anything which more closely matches what you're looking for.
