Thanks EmyrBorth, that's really helpful. I've actually now got (what I believe) is all except the 1871 censuses for Laura, though the early ones are a puzzle, as is the whole family.
She appears to have been born in Aberech as Laura Rowlands, daughter of William and Ann (nee Jones), but recorded as Laura Rowland in Llanengan in 1851, and as Lowry Owen at Cefn Coch, Llandegwning in 1861, with parents William and Ann Owen, and siblings all listed as Owen. I had the parents and several children (but not Laura) in 1871, all recorded with the surname Williams, still at Cefn Coch. William and Ann were living there (as Williams) until their deaths in 1905 and 1907 (William died at 9 New Street, buried at Llandegwning).
Are you able to tell me if John and Laura's son Ellis John Roberts (died 1914 at 2 Glaslyn St, Portmadog), and his wife Martha died in 1944 (then married to Edward Parry), and their daughter Laura Augusta (died 1932) are buried there too (I like to tidy things up if i can

)
I've been trying to trace all William and Ann's children in the hope of finding out which of their daughters (at least 6) was the mother of William, who was aged 5 in 1891, living with his grandparents at Cefn Coch. Several of the daughters appear to have had children but were not married, so a bit of a needle in a haystack.
Again, many thanks, Deirdre