At that time your ancestors could have been going to chapel rather than church, because of the Welsh Methodist Revival of the 18th century. Llangattock indeed is a rural area now, but at that time there was a lot going on in the area, with the quarrying of stone from the mountainside, the building of the Brecon & Monmouth Canal, and various industrial activities going on in the valleys on the other side of Llangattock mountain. If your ancestors were chapel, then there would not have been any baptism recorded in parish records. Not sure if chapel records are available for that period, but it certainly was an interesting time historically, with a lot of people moving from place to place, seeking better wages and more comfortable housing conditions.