Pat, do you have Dorothy's birth/baptism record? Is this where you got her parent's names from?
I would check for any older or younger siblings with the same parents to get an indication of where and when Thomas and Mary may have married. This at least might confirm the one you found in Simonburn.
Also, the Parish Records of the children's birth/baptisms may say where Mary is from, so check the records of all of Dorothy's siblings.
I'm not sure if these dates are covered in the appropriate parish, but it might be worth looking on the familysearch site at the Durham and Northumberland BT's.