Sorry about delayed response, been ve-ry busy etc. In answer to your question:

1) I double clicked in footer area & it become visible (with main text greyed out)

2) entered footer text then, design tab > header & footer group > page number > bottom of page > page x of y

I've also tried it by
Insert tab > footer > blank > enter text in field then > page number > bottom of page > page x of y

Both ways have the same effect on the undo function.

I've just discovered this: http://tinyurl.com/d4tx8x7 I think it’s a post if yours on another forum, or someone with the same name. It seems to be related to some types of page numbers, but not others!?

Via the link above I found this http://tinyurl.com/clzfu2r, but the suggested solution doesn’t seem to work.