Looks promising. In searching through the IGI I found a Catherine Cubbon born about 1766 in Bradden whose spouse was William Quayle. You show her birth as 1771. I did find their marriage as July 7 1787 as you stated but she would have been 21 the same as William. Where did you get her birth as 1771 as I couldn't find it.
Also Catherine Quayle (my gggrandmother)whose spouse was Charles Brew shows on the IGI as being born abt. 1805 at Marown. If she is your ggggrandfathers younger sister there are two IGI's on her. One shows her christened on March 23 1806 in Marown and the other March 23 1806 in Braddon; with both having the same parents; William Quayle and Catherin Cubbon. Where do these IGI records come from and which one do you believe if any?
I also found it interesting that I came across a Kath Cubon born Feb 12 1764 in Braddon with a Margaret Brew as her mother. As my Catherine Quayle married a Charles Brew there could be a connection. I had estimated that Catherine Quayles mother was born in 1764 which makes it all the more curious.
Any further thoughts Colin?