Bow,
this link should take you to the N Wales BMD site. It is especially useful because it is based on the local (and so original) civil registers rather than the copy returns sent in to the GRO, and it gives the parish or sub-district of the event, allowing more reliable matching.
Peter, on looking at that 1857 marriage in the name Sinah Hughes on NW BMD, the site did not seem to offer me a matching groom called Thomas Davies -- instead I got a John Owen, plus the Thomas Jones you found, both shown as marrying some time that year at Cerrig church. NW BMD seems to have two Thomas Davies marriages during 1857 in parishes that fell within the old Corwen RD, one at Corwen church and one at Llangollen church -- and these are presumably the two that come up on FreeBMD in 1857 for that RD, i.e. the one you found in Q2 and another in Q4 (the latter apparently the Corwen one).
Looking at the 1871 census for Hafod-y-maidd township at Cerrig I see that there is a John Owen in an ag lab's cottage there with a 35 year old wife called Sinah (both natives of Cerrig): RG10/5680 fo.92r p.7. As their eldest child was born 1858-59, it looks rather likely that they are the right pairing for the 1857 marriage at Cerrig church -- so it was probably not the wedding of Bow's Sinah. None of which is any positive help I am afraid . . .
I do see that in 1874 NW BMD offers another Sinah Hughes marriage, at Llangwm church and with a David Jones as the only match. But there is a fair chance that the census removes them as candidates too, if they are the couple aged 31 and 27 in Voel township, Cerrig, in 1881 -- as seems quite likely: see RG11/5539 fo.33r p.1.
Accordingly I am so far able to offer pretty little on Sinah's fate. There must be the usual risk of mis-enumeration at the time, or mis-transcription into the modern indexes. I did try looking for all females stated to have been born "Denbighshire" + "Llanfihangel*" in 1840, ±1 year; and if she was mis-enumerated as "Sarah", there is a long-shot candidate in 1861 -- a Sarah Hughs, unm., 21, b. Denbighs. Llanfihangel, who was working as a general servant for a draper in Whitford Street, Holywell: RG9/4269 fo.31 p.56. But NW BMD is rather discouraging about the possibility of such an error, because it shows two Sarah Hugheses as born 1840 in the right sub-district (Gwyddelwern) for Llanfihangel GM births . . . Rather interestingly, while Sinah Hughes shows up in the GRO birth index in Q3 1840 (Corwen RD), NW BMD gave me no Sinah Hughes births anywhere at all from 1839 to 1842 (even on soundex). Bit odd.
Rol