Investigating members of the Sharp family in north London, there was a witness with the somewhat unusual forename Rosina (1852). I looked her up on familysearch.org and discovered that she was possibly born in 1832 the daughter of John Sharp (expected) and a Hannah Rosina Sadler (not previously known) (born possibly c 1803 according to FreeBMD which records her death at Kensington 1891).
This rang slight bells - and looking through my records I found that a possible granddaughter of John and Hannah was a Louisa Seidler Sharp mentioned in a 1913 will.
Further browsing through my notes suggested a hitherto unidentified German branch.
I am left with a suspicion that myHannah was originally Seidler and (extending my musing) that she came of a musical family - subsequent Sharps were in this business.
Could she have been descended from a Hanoverian soldier or a German refugee from Napoleon?