[Exim] strange behaviour with .forward files

This is with Exim 3.02.

I have a director:

backtoexacuk:
driver = smartuser
domain = user
no_rewrite
new_address = ${local_part}@???
new_director = system_aliases

anotherdirector:
...
...

system_aliases:
driver = aliasfile
...

userforward:
driver = forwardfile
...

localuser:
etc.
etc.

The problem is:
For an address in the "user" domain, the first time through the path is into the
backtoexacuk director which sets up the new address and then to the system_aliases
director and then to the userforward director. Here the user has "\username" in
the .forward file, so that is picked up and we start again. When it gets to the
backtoexacuk director again Exim says "backtoexacuk director skipped: previously
directed to username@user" and it passes on to the "anotherdirector" director
instead of going the same way as the first time! I can see that the address must
not loop, so it should not look in the .forward file again, but if it had not
found a .forward file in the first place it would have dropped through for local
delivery. Now it gets munged by "anotherdirector"!

I fear I am not fully comprehending the basic principles here!

Phil.
---------------------------------------
Phil Chambers (postmaster@???)
University of Exeter