Clause Complex With Ellipsed Subject

Canterbury Cathedral	was	founded	in AD597	and	Ø	is	the Mother Church of the Anglican Communion
1				+2
Theme	Rheme				Rheme
Goal	Process		Location		Token	Process	Value
Subject	Finite	Predicator	Adjunct		Subject	Finite	Complement

Reasoning:  Ellipsis marks elements as textually non-prominent, whereas Theme marks elements as textually prominent.  Therefore, ellipsed elements do not function as Theme.

Halliday & Matthiessen (2004: 563):

Ellipsis marks the textual status of continuous information within a certain grammatical structure.  At the same time, the non-ellipsed elements of that structure are given the status of being contrastive in the environment of continuous information.  Ellipsis thus assigns differential prominence to the elements of a structure: if they are non-prominent (continuous), they are ellipsed; if they are prominent (contrastive), they are present.  The absence of elements through ellipsis is an iconic realisation of lack of prominence.