Not sure if anyone can help but here goes:-
An Elizabeth Jenkins (probably baptised around 1755 in or around Begelly, Pembrokeshire) married a Gwyther c 1778. They had Margaret, who married James Aris of Horsleydown, Surrey, George who married Henrietta Ann (Countess of Rothes), Martha (my 3*gt grandmother) who married Thomas Dunnell & John Gwyther who married Elizabeth Probate or at least I think they did!! George Gwyther appears as a witness on the marriage entry of Margaret and Martha and the signature appears to be the same. He was in partnership with Thomas Jenkins, his uncle, as Jenkins & Gwyther and they were nursermen in Marylebone as well as Regents Park. How George met Henrietta Ann I do not know but the married twice! Once in 1806 in Deptford St Nicholas and again in 1807 in St Marylebone.
Is anyone else researching these Gwythers who would like to share information? I have pulled together quite a reasonable selection of information but I could really do with more substantive evidence of Martha's parentage as well as the rest.