Hi there, Alun
This is what I've found. The marriage still remains a mystery but!
1861 census Merthyr T. 7 Glantaf Rd.
Daniel 1832 (LLEWELYN) woodcutter
Ann 1831
Elizabeth 1855
Jemimah 1856
Evan 1858
William 1860
1871 census (LLEWSLLIN)
Daniel age 40 born Llanwonno
Ann 40 Kendle, Ebbw Vale, Mon.
Jemimah 15
Evan 13
William 11
Mary Ann 4
Daniel 2
1881 census (LLEWELLYN)
Daniel woodcutter and stone quarryman
Ann
William 1860
Mary Ann 1866
Daniel 1868
Catherine 1873
1991 census
Catherine was living with hermarried sister
Mary Ann Walters married to Morgan (that's what it looks like)
Familysearch.org gave these results
Ann Davies christened 23.12.1832 at Bethesda Welsh Calvinistic Methodist, Kendle, Monmouth
father Rees Davies
mother Rebecca Prosser
Daniel Lewelyn (diff spelling again)
born Llanwonno, Glamorgan 20.07.1832
Father Evan Lewelyn (Daniel named a son Evan)
Mother Jemima Mathew (Daniel named a daughter Jemimah)
married 5.10.1819, Llanwonno, Glamorgan (LEWELIN spelling for marriage)
Hope this is of interest. I'll have another dig for the marriage. Being that they had Elizabeth in 1855 possible marriage is 1854. As they were living in Merthyr T in when Elizabeth was born chances are that'll be the locale of marriage.
Regards
H