Whilst only half of the births registered in Biggleswade baptist church register named parents Richard and Elizabeth, the fact that Frances born 18 Jan 1789 per the Baptist records was baptised in Stotfold on 27 Apr 1806, with a birth date of 8 Jan 1789, links the Biggleswade Richard and Elizabeth with the Stotfold Richard and Elizabeth. Whilst not cast iron proof I find it pretty conclusive.
The 1813 reference was from Samuel Whitbread's Notebooks. He was the local magistrate. There's an Elizabeth Charles age 60 buried in Stotfold on 22 Dec 1813, which looks about right datewise, bearing in mind that she was having children until 1796. Age 78 in 1813 can't work as she'd be too old to have had a child in 1796. On baptism in 1764 Richard was described as "about 6". The whole family seems to have been baptised together, including Elizabeth, widow, which is odd as there's no burial of a John until 26 Feb 1770, Labourer.
David