Keith, do you know the biological parents of your great-grandfather? If so, I find the easiest place to gather clues (note I call them clues, as they require research to prove) as to the lineage is to search on ancestry.com (if you have an account). You mention Dutch, German, English. Was he born in the U.K.? You also can do a free search on familysearch.org.
If you don't know the biological parents, then you need DNA to figure out the lineage.
Give us a bit more to work with and we can help you figure out the best method(s) to use. On that note, I have added adoption tags to your post to get it in front of the right people.