Hello all; would anybody be kind enough to critique my use case diagram below? I am a basic UMLer and trying to improve. I have been working on a use case diagram for Manage Invoice which involves representing Create Read Update Delete (CRUD) appropriately as well as factoring in a one (header) to many (items) in data terms. I think this diagram is ok ?? but would much welcome comments. Many thanks in advance. David.

PS so sorry but I can't work out how to provide a link to a read only copy of the diagram so here below is a description of the diagram and a screen shot further below. Please let me know how to create a link to read only copy if you would be so kind... !

Description of Manage Invoice use case:
Invoice Clerk associates with use case Create Read Update Invoice Header
This use case Extends to Create Read Update Invoice Item/s and also to Delete Invoice Header
Delete Invoice Header Includes Delete Invoice Item/s
Create Read Update Invoice Item/s Extends to Delete Invoice Item/s.  

Here below is the screen shot...