Thomas Howells(died 1813 was ca. 76 years old) when your Wm.Howell was born. The best fits of yours in Defynog registers would be-
William s/o Howell & Barbara Powell
(rec.41.173, he a cooper) chr. Oct. 4, 1812.
or
William s/o Thomas Powell & Mary, chr. S.25,1806.
In my opinion the best bet would be he chr. Oct.4,1812.
Does the old part of "Birchgrove" have small windows, walls mostly rubble built(maybe rendered now)& a roof much stepper than the newer part?
I have one a recon.(not finished) of Thomas Williams ="your" Gwenllian James, lived at Llanigon '51-'61.
Thank you for the possible offer of help but what do you have in mind as available.
Garfydd