If you're plotting your own family tree, come and see how Sir John St John did his. The magnificent 17th century St John polyptych will be open today revealing the St John Tudor connections.
Come and marvel at the 14th century wall paintings, the medieval glass and the stunning St John Bed Stead memorial.
Then join me on a guided churchyard walk where I will tell you about the ordinary country folk who have worshipped in this fascinating church.
Meet Jonas Clark who lived with Alice Pinnell for more than 30 years and had seven children before he could marry her.
Why was Martha Hale buried in Radnor Street Cemetery, Swindon and not with her husband Charles here at St Mary's.
And what is the significance of the butterfly on Vernon, Viscount Bolingbroke's headstone?
The church will be open from 2pm and walks take place throughout the afternoon.