only death registered at Crickhowell in the right period, no ages given on index at that time, so can't be sure it's the right person
Richard Evans march qtr 1864 Crickhowell vol 11b pg 107
Sorry, don't understand why you say children & ages for Elizabeth don't match. They do match for the census info & birth registrations in Wales.

Elizabeth in 1851 census
Gellygare HO107/2457 folio 188 pg 31
John Hereman head 62 iron miner b. Silworthy, Somerset
Mary wife 60 b. Llantwit Major, Glamorgan
John son 21 iron miner b. Gellygaer
Elizabeth dau 15 b. Merthyr
Baptism in Merthyr Parish Church 1 Nov 1835 for Elizabeth, daughter of John & Mary Hereman, John noted as miner and their abode Dowlais.
The enumerator did a good job of hiding them in the 1871 census
Llangunider, Breconshire RG10/5584 folio 90 pg 17
Thomas Thembo head mar 28 coal miner b. Tenbury, Worcestershire
Elizabeth wife 35 b. Dowlais, Glamorgan
Mary Evans stepdau 15 b. Aberdare, Glamorgan
Jacob Evans stepson 12 b. Aberdare
Margaret Evans stepdau 10 b. Aberdare
Susannah stepdau 8 b. Llangunider
John Thembo son 5 b. Llangunider
Sarah J dau 2 b. Llangunider
There are later marriages for a Sarah J Bembo/Benbow, but bit late for the one born around 1869, and not to a Thomas Williams. One in 1916 to William H. Williams.
The cheapest option is to order certificates direct from the GRO & pay with a credit card
http://www.gro.gov.uk/gro/content/certificates/