« Reply #1 on: Saturday 08 November 08 00:25 GMT (UK) »
it's only the 1851 census where she shows up as Winifred. She's Gwenllian in the other years. Her parish of birth appears to be Llangeinor.
There is a marriage that would fit - Morgan Williams of Llangonyd to Gwenllian Bevan of Llangeinor in Llangeinor 23/4/1831. The two places are only a few miles apart.
http://www.genuki.org.uk/big/wal/GLA/Llangeinor/You can get a copy of the parish entry from the record office in Cardiff
http://www.glamro.gov.uk/repro%20of%20rec.html Do you have birth certificates for any of the children born after the start of civil registration? It would confirm mother's maiden name.
Beware of the submitted records on the IGI, they need to be verified and treated with the greatest suspicion. The dates for some relatives of my mother on a submitted record are all 10 years earlier than they should be and we're not talking early records here, but ones from the 1900s. Easily checked on the GRO index as both first name & surname are unusual, but no, the submitter has made the whole family 10 years older than they were.

Cornwall: Allen, Bevan, Bosisto, Carnpezzack, Donithorn, Huddy, James, Retallack, Russell, Vincent, Yeoman
Cards: Thomas (Llanbadarn Fawr)
Glam: Bowler, Cram, Galloway, James, Thomas, Watkins
Lincs: Coupland, Cram
Mon: Cram, Gwyn, John, Philpot, Smart, Watkins
Pembs: Edwards (St. Dogmael's)
Yorks: Airey, Bowler, Elliott, Hare, Hewitt, Kellett, Kemp, Stephenson, Tebb