Cannot make Emacs honour TeXInfo mode or filter

I am trying to use ispell-mode in Emacs to spell check a TeXInfo file.
I have never tried this before, so suspect I am making a stupid
configuration error somewhere, but I am stumped.

From the command line things work fine.  Something like:

$ aspell check org.texi

does exactly what I would hope, skipping past the first \input texinfo
line and other TeXInfo commands nicely.  I do not have much in
.aspell.conf apart from a few lines like:

add-texinfo-ignore set
add-texinfo-ignore macro

which also work well.  I do not have a mode option set,  Aspell seems to
recognise the .texi file extension.

In my .emacs I have:

(setq ispell-really-aspell t)
(setq ispell-program-name "aspell")
(setq ispell-help-in-bufferp (quote electric))
(setq ispell-personal-dictionary "~/.aspell.pws")
(setq ispell-silently-savep t)

However, running M-x ispell-buffer on org.texi stops immediately at the
\input texinfo line wanting to correct the word texinfo.  It seems that
in this case aspell it does not recognise the file type.

I have also tried 

(setq ispell-extra-args '("mode=texinfo")) and also setting
ispell-extra-args as a file local variable with 
ispell-extra-args: ("mode=texinfo")

but that doesn't seem to make any difference.

If it may help:

$ aspell version
@(#) International Ispell Version 3.1.20 (but really Aspell 0.60.5)

$ emacs --version
GNU Emacs 23.0.60.6

$ uname -spr
FreeBSD 6.2-RELEASE-p7 i386

Does anyone know what I have done wrong?

Thanks

-- 
Phil