Note sure if the reply came to this list also so I will post it here also.
Have you researched the google books site?
http://books.google.com/booksThere is a book called THE GENEALOGY OF THE PURCHASE FAMILY in Britain and Southern Africa. You can pull it up by title... a portion of the book is there in preview and might help you locate origins of the surname prior to 1750 (actually back to 870 A.D.) Very interesting read. The first few pages are online and a statement is indicated that some of the records were burned that marked the Christenings from 1689-1690 and more. But what is there is worth reading.
Leslie Barker Thomas