How do I get global-grep-and-replace to work?

[oisin@ukfsn.org (Cinnamon)]

Hi,

I've a large number of files that need one line changing.

I tried global-grep-and-replace, which asks me what I want to change,
what I want to change this to, and then asks me which files.

I get a neat buffer with all the offending lines in all the files
listed, then it askes me Do you want to [replace, edit, search, quit]:

When I say replace, it moves to the first buffer onto the line, but
instead of changing those lines for me automagically, it just sits
there.  Also, I'm not sure why its actually going into the file and
showing me -- I already said to just replace this with that in *.c, so
no need to show me the files, just go ahead and do it :)

What is going on?  And is there a way I can just coax Xemacs into
changing the lines in those files without any more work for me?

thanks,

Cinnamon