Are you saying that your father and grandfather have previously researched your family history and got back to Nathaniel Haggard b 1687 and earlier ancestors?
If so, it might be wise to double check all their research before you attempt to go further back. It is always good to make sure that the correct lines have been followed, as it is so easy to make one minor error and follow the wrong family.
Even if it was family members who did the research they could still have made mistakes. These days there is a wealth of information available online which was not around a few years ago. It would be relatively easy to double check their findings, buy a few certificates where necessary, and run any queries past the people here on rootschat, just to make sure that your father and grandfather did get it right.

That is how I would approach it anyway.