Thanks for your reply. I had thought of your second explanation since my own grandfather and his two brothers have the "middle name" Catchpole on their birth certificates (late 1800s) but the surname Sandford. Their father was married to someone else at the time of their births (she had TB) and father and mother married after she died. However, the brothers didn't use the term "otherwise" and I believe just adopted the name Catchpole without ever using Sandford.
As far as Ann Littler is concerned I thought perhaps she had been an orphan or adopted (but since there was no formal adoption process in the UK I doubt that). On her marriage certificate a witness is Mary Littler, so do you think Watson was Ann's original surname or Littler?