Hello,
I am distantly related to this Bull family. At present I have only done cursory census research to prove the link.
John Bull and Mary Shelley actually married in Boldre according to entry 178 page 60 parish of Boldre 04/09/1832
Their son John c1844 marr Thyrza Bell from Whiteparish Wilts. His brother Samuel Bull married Thyrza's sister Vashti Bell.
John Bull and Thyrza's son, James Walter 1869 married my kinswoman, Kate Elizabeth Bunce b 1872 Bradfield Berks, dau of Joseph Bunce and Jane Sellwood, and moved to Putney.
Jane Sellwood is my link being one of my paternal Sellwoods.
James Walter Bull and Kate Elizabeth had 2 children, one of whom was killed in WW1.
That is as far as I have got but I would be happy to share what I have researched in exchange for any other information/photos/etc.
Vicwinan