A search through the telephone directories will find around 300 Whitmarsh names. I am currently searching through the Birth Marriage and Death indexes at the Family Research Centre in London. I am about half way through (the period 1837 to 1950) and have found around 2.500 names. I estimate there are 5,500 in total.
My late father traced our tree back to Hampshire in around 1700. I would be interested to find out about your family trees.
I am not aware of any possible connections to other branches since the end of the 19th century. My g.g.grandfather had a brother who had a couple of children but contact was lost when the mother ran off with the music teacher!