Suz,
Yes, this could be possible, we've found 4 daughters, by the looks of it. Maybe Sarah Haynes died and then he married a Mary, who was considerably younger than William.
We need to find a marriage now for William Haynes and Sarah, as well a Mary, will start checking myself.
It seems that the Haynes family moved from Dewchurch, Lanwarne area to Monmouth, lived there for a while.
The marriage certificate for Thomas Gwillim and Susan Haines shows her signature as Susannah,
but certainly registered as Susan, maybe she wanted to be a "Susannah" Have found her surname as Haynes in her son's birth certificate.
jan