auto-insert-mode template format

[Gary .]

I know I can use skeleton files, and that works okay, I just find the
syntax less easy to read than a simpler one would be. I found by way of
the emacs wiki a document which I thought explained the "simple" format
of auto-insert files
(http://web.archive.org/web/20040604170543/http://www.cogsci.ed.ac.uk/~tkld/auto-insert_toc.html)
but I'm damned if I can get the %-escapes described there to expand.

For example the config:
(define-auto-insert "\\.foo$" "insert.foo_")

where the file "insert.foo_" looks like:
//
//  Filename        : %f
//
doesn't replace %f with the file name as I expected.
(note: it also doesn't work surrounding the %f with `' as indicated on
that webpage, I tried it just to make sure)

Any ideas what I am doing wrong?