what occupation is given for John Nicholls on the marriage cert? It's not an uncommon name. Is there an address given for Katie? If so, have you looked at that address in 1881 to see if her family are there?
Her age is given as 26 on the 1891 census & 36 on the 1901, so it look like she added a few years at marriage as her husband was older.
The father of Katie Wood Nichols was a farmer and was baptises in Lostwithiel, some distance from Redruth.
There is a possible for Katie on the 1871 census in Taibach with father John, a collier RG10/5423 folio 59 pg 14. That entry gives her place of birth as Gwennap, next to Redruth.
http://www.cornwall-opc.org/MAPS/parish_map.pdfThere's a matching family in Gwennap in 1861, but John is a mariner.
If this is the right family, there are possible birth registrations for 2 of Kate/Catherine's older siblings, but with her varying age, it's difficult to be sure which one is hers.
John Henry Nicholls june qtr 1856 Redruth vol 5c pg 287
Emily Nicholls sept qtr 1858 vol 5c pg 257
