I've been using ancestry too - so you got the marriage from that site? I must be going blind because I spent a while looking through all the records. Although I was imagining that he was married a bit later mid 1840's and I was probably thinking of the wrong region.

I think that this is the birth cert for Mary Powell -
Merthyr T, Sep 1851.

Sometimes it's easier searching for birth certs on freebmd I think.