I have heard of the Fenn family of Newgate Street. They were watch and clock makers and makers of lathes and other clock making tools.(See http:www.workshopman.co.uk/uphill and_son/pages/ot/Fenn/about-fenn.htm) I thought that this family might be related to my Fenns, as my gr gr grandfather John Fenn (Dob 1814 Deptford Kent) was a watch maker in the London area, as were 3 of his sons, a nephew and 3 grandsons. However, I now believe that Newgate family and my own Fenns are not related. The founder of the clock making business at Newgate Street was Daniel Fenn who wes born in Bedford in around 1711 and was apprenticed to Henry Thornton in London in 1725.Daniel's sons and grandsons continued the business in Newgate Street until 1868, when the shop which projected by the railings of Christs'Hospital, was pulled down. Daniel and some other members of his family were Masters of the Worshipful Company of Clockmakers of London.(See www.clockmakers.org and www.clockmakers info/page14.htm.) Daniel married Mary Jenkins at All Hallows London Wall in 1738. They had at least 10 children, some of whom are listed on the IGI. Daniel's will shows that he was very wealthy and owned a country estate in Leyton Essex.The will also mentions his sons John, Daniel, Samuel, Joseph. James & Thomas.In turn John's will makes no mention of children whilst Daniel junior's made his will to his step daughters.It seems that one of the other sons was your Mary's father.The wills can be found and downloaded for a fee from www.nationalarchives.gov.uk I have also found on Ancestry an inscription which relates to your family: Middlesex Clerkenwell Monumental Inscriptions of Charterhouse Chapel. Burial of Brethen & others from 1/1/1754 16/09/1803 Thomas Fenn Ramsden, son of Thomas & Mary Ramsden of the Royal College of Physicians was buried in the Cloisters leading to the chapel, died 13/09/1803, aged 8 months. It appears that the family were Non Conformists.
I hope this information is of some help to you. I am sorry that I cannot be more precise about your Mary Fenn's father.