[Exim] String expansion for list-id: header

Hi Guys

I'm having trouble getting exim to expand the list-id header for
.forward file filtering, an example filter is given below:

## HLDS
if $header_list-id: contains "exim-users.exim.org" then
        save $home/Maildir/.exim/
        finish
endif

Having tested it, it doesn't work for me, however string expansion on
subject, from, to etc all work fine. Have I mis-understook what it is
possible to expand in headers ?

Thanks in advance

Simon