Sid CD fails as apt source

[Andrew Malcolmson <andzy@imap.cc>]

I'm trying to use a sid iso image as a apt source.  I've successfully 
mounted it and added a file entry to sources.list as:

deb file:/mnt/iso/debian sid main

When I run apt-get update, apt reports it can't find Packages.gz:

Failed to fetch 
file:/mnt/iso/debian/dists/sid/main/binary-i386/Packages.gz  File not found

This isn't surprising though since Packages.gz on the iso is in the 
sid/main/binary-alpha directory instead of in sid/main/binary-i386 where 
apt expects it to be.

I could copy the image to my harddisk and rename the directory to 
binary-i386 but I shouldn't have to do this, should I?  I must be 
missing something simple here.

Thanks