There is no place or date of their marriage listed in Civil Registration for the U.K. All mrge in England and Wales (in theory anyway) must be registered within 6 weeks of the event. I will not go into long explanations why, but the Government of the 1850’s introduced much heavier penalties for non registration or non-compliance of this law introduced in September of 1837. Religions actively against registration of the time included, Jewish faith, Quaker Faith, and some N-C faiths (that I know of), who believed that the Government had hidden reasons for this law of registration. Would you agree that your John Henry and Margaret are listed at Mount Pleasant in the 1861 ; RG 09 Piece 4113 Folio 54 Page 45 Schedule item nbr 261 (not house nbr). This only lists Martha aged 2 yrs and Thomas aged 1 year. Since it does not list William Evans and there is only listed after the date of the 1861 census of 02nd Apr 1871 it would be safe to assume that “your” William’s birth was registered in the 2nd Qtr of 1861 as per Free BMD. Since you know his parents the only extra info gained would be address at time of birth. Remember U.K. civil registration is index’d by date of birth registration and never by date of the birth. Registration of birth (by law) has no connection whatsoever to date of chapel admission or established church christening (by personal choice) Given that Martha is aged 2 and Elizabeth Samuels is aged 23 in 1861 this now limits (IMHO) to the period of 1855 to 1859 for a mrge of John & Margaret Elizabeth, in which case we are left with only one entry in Civil Registration and that is of John Evans to Elizabeth Samuel in the Merthyr Tydfil area in the 3rd Qtr of 1856 Vol 11a Page 402. Before you make the comment on forenames, this is not the 1st time by any means that I have noted and confirmed that one or either party has used only their 2nd christen name or missed off any part of their other christen names. However final decision if of course yours. Given my above findings above I best I can suggest is that John Henry Evans was the son of William & Margaret Evans mrd at Llanelli 27 Sept 1825. Given the above the best I can come up with is Margaret Samuel christened at Llangennech (very near Llanelli) on the 31 March 1833 to John & Jane Samuel, and that John Samuel had married Jane Howell at Llanelli on the 04 Sept 1815.
Margaret Samuel born 1832 (as per 1841) is the daughter of Thomas Samuel who married Margaret Hugh at Llanelli on the 25th Nov 1828
I do note out of interest that ; John Samuel died 14 August 1870 a Butcher of Mount Pleasant (see Census above) his will was proven 01 Nov 1870 to Eleanor of Mount Pleasant widow the relick of John.