Well, after googling the names 'Thomas Pettit and Mary Markwick' I'm of the opinion that this couple aren't ours. A very extensive research site has Thomas marrying Mary Markwick but not only that, he also had 3 other ladies on the side. Elizabeth Botten, Jemima Wells and Kezia Sharpe. All known children of Thomas to these ladies are listed and there appears to be no James. Seems Thomas was quite well off as most of these offspring are mentioned in his will. Have a google and a read. It's quite interesting.