Hello Calvin -- my apologies for posting info you already knew. My confusion stemmed from the fact that your initial query stated that you "believed" that your grandfather's parents were John & Sarah Jane. I mistakenly took this to mean that you weren't sure, ie. hadn't confirmed the fact that they were his parents.
Anyway, good luck in tracing descendants of this family. This can be more difficult than tracing ancestors of centuries ago because, for example, there are no censuses or even parish records to rely on. One option is to try and trace the births, marriages and deaths in the family via the BMD civil registration indexes.
Another tactic that has worked for me is to write Letters to the Editor in newspapers published where the family is known to have lived. These queries are more likely to be published in smaller towns, though. Newspapers in bigger cities tend to reject such letters or suggest placing adverts in the classified section. Still, worth a try.
You could also try tracing/finding the family in phone directories although a surname such as COLEMAN is fairly common -- and judging by the 1901 census, first names were common ones as well -- so it may be difficult and/or impossible. For example, the Ancestry.co.uk website is now offering a database of British Phone Books dating from 1880 to 1984 as well as a 14 day free trial offer subscription. As an alternative, many libraries and archives in England have old telephone or residents' directories for their particular geographical area.
Lastly, have you obtained all of your grandfather's military papers and records? These may give you some clues, particularly of family members around the time of the Second World War. Good luck and regards, Andrea (Kent Board admin)