A researcher from University College London kindly sent me a copy of a sketch labelled "Mary Tonge", in the hope that I could help identify who it was.  It was found in a collection of manuscripts relating to Sir John Bowring who was, among other things, MP for Bolton from June 1841 to 1849.

Permission to reproduce this image has been kindly granted by the Director of Library Services, Brougham Collection, UCL Library Services, Special Collections, 25th May 2006

I would estimate that the lady in the picture is between 15 and 30 years old.  If we suppose that the picture was drawn during Bowring's time at Bolton (1841 - 1849), that would give a date of birth of between 1811 and 1834.   The 1841 census shows us that there were at least 26 Mary Tonges in the Bolton area at the time who were in the right age range.