A coupl of mistakes you have made here, because this is a Gloucestershire related family. Monmouth registration district covers the North end of the Forest of Dean, So the marriage probably took place in Gloucestershire an easy mistake to make.
Monmouthshire registration districts
http://www.fhsc.org.uk/genuki/REG/mon.htmGloucestershire registration districts
http://www.fhsc.org.uk/genuki/REG/gls.htmThe other mistake is to take for granted that your findings on the civil registration indexes are correct. you are probably right with Caleb Aston being and unusual name, but there again they may not have married each other and she also may be a widow.
Until you purchase the marriage certificate you cant be sure. it would also help when asking other people to find someone for you having the fathers name from the certificate, because there again they could be following your mistake.
The Gloucestershire 1851 census transcription.
http://www.rootsweb.com/~cotswold/GLSontheweb.htmI have Aston & Worgan pedigrees from this area in my database.
Mike John