Best thing to do would be to get her birth certificate, then try to find her parents' marriage and where they were in 1901.
I can say that 'Trevorthan' is a very Cornish name, possibly the maiden name of one of her grandmothers. Could be a varient of Trevarthen or Trevethan, or something similar.
I did try searching freebmd for Fisk m. Richardson: no likely hits (only two where the Fisk was the wife, it is likely the marriage is not on that index yet)
There are only two male Fisks b. Cornwall on the 1901 census, and few living there: there is a family in Cambourne:
Wiston Fisk (25, b. Blythburgh Suffolk)
Annie (27, b. Cornwall)
Arthur, (7M, b. Cornwall)
In E. Looe:
Albert Fisk, 57, b. Cork Ireland.
over in Plymouth, Devon:
Thomas Fisk, 29, b. Looe (son of Albert?)
Elfrida's father may have been not originally from Cornwall, but married a Cornish girl (is there any navy/merchant navy link in this family that you know of? Or fishermen, coastguards, etc...) and that was where the Trevorthan link came from.
