I've got a document, 25 pages long, but 57 including attatchments that fit in at various points. How do I get it to skip pages 23-48 (for example) in the page numbering in the headders?

Section breaks are your friend. If you pop in a section break at the end of the initial block, then after the section break go in and change the page numbering in the header and you should find the option to format page number, in there you will get the option to restart page numbering from any number you choose. You can faff about with codes and the like, but not needed if it's a one off.

As MLC says.

There's more here. You want Big idea#5
http://word.mvps.org/faQs/Numbering/PageNumbering.htm