For what it's worth,1841:
http://search.ancestry.co.uk/iexec?h...=&pid=16136479
I see Rachel is a lodger in 1851, but didn't look for Ann jr or Thomas.
There's an absolute minimum of two good matches for the Mo-in-Law, Elizabeth Thomas, in 1841 within five miles of the 1951 addy.
You have my sympathies with the Welsh research. I recently purchased four birth certs to help a lady with her tree (all t discover the mmns). Three of the four birth certs were for a child named Jones. Two of the Jones birth certs turned out to be mmn Jones as was the case with the non-Jones cert. Two mums and two dad's had exact matching names. It is so confusing, because it's impossible to remember any of the tree.