Hello Mabel!
Thank you for so thoroughly researching those documents at the NLW. I subsequently reviewed them myself, and didn't find another reference to Jenkin Williams, so I went back to have another look at Thomas Williams will of 1792.
It states that he owned an property called the Willbrook Estate in the Parish of Peterchurch Hereford. It was to become his after the death of his mother Bridget Williams. Thomas instructed "John Hughes and Hugh Bold" to SELL Willbrook as soon as he that happened. The Proceeds were to be applied to repayment of a 2000 pound mortgage that he had previously obtained from Jenkin Williams.
It is possible that Jenkin was living at Willbrook with his mother in 1792, and might have acquired the estate after she died. Dorstone is a couple of miles North of Peterchurch, and Willbrook Manor is a couple of miles South.
Judging from the interior of Willbrook Manor (see attached), Jenkin would have been well advised to move elsewhere, so it is highly likely that he moved to Dorstone, where his will was made.
There is a good description of Willbrook on the Web, it is currently owned by the Vivat Trust
So I think this is the Jenkin Williams, brother of Thomas Williams Skreen and Velyn Newydd.
Thomas seems to have been in a financial mess when he made his will in 1792, and I think it was the beginning of the end of the Family estates. Thomas had a liability of 2000 pounds to Jenkin, at the same time bequeathing thousands to his sons John and Thomas and two daughters Elizabeth and Bridgett.
There is no mention of Jenkin Williams children anywhere on-line that I can find, so his son Thomas could well remain a mystery.
Thanks for your help... it provided me with a lot more information on the Williams family of Skreen etc.
My very best regards
llanfi