exim configuration for tmda

[Jeff Penn <jeff+dated+1056398094.1d9d93@jrpenn.demon.co.uk>]

I'm having problems configuring tmda to run on Sarge with the default
exim install.  I've followed instructions in the tmda manual except:

echo "|/path/to/bin/procmail -p" > ~/.forward

I left this file empty since procmail is specified in exim.conf by 
default.

All mail ends up being passed to procmails $DEFAULT,

~/.tmda/logs/debug contains:
Traceback (most recent call last):
  File "/usr/bin/tmda-filter", line 53, in ?
    execfile(os.path.join(execdir, 'tmda-rfilter'))
  File "/usr/bin/tmda-rfilter", line 175, in ?
    raise Errors.MissingEnvironmentVariable('SENDER')
MissingEnvironmentVariable

Which appears to suggest exim is not passing procmail the required
environment variables.  I have tried various changes including modifying
the procmail_pipe configuration in exim.conf without any success (such
as adding -p option to the command).

/etc/exim/exim.conf:
trusted_users = mail:uucp:jeff 
  
address_pipe:
  driver = pipe 
  path = /usr/bin:/bin:/usr/local/bin
  return_fail_output
  return_path_add
  environment = EXTENSION=${substr_1:$local_part_suffix}:\
                RECIPIENT=$local_part$local_part_suffix@$domain
  
procmail_pipe:
  driver = pipe
  command = "/usr/bin/procmail"
  return_path_add
  delivery_date_add
  envelope_to_add
  suffix = ""

userforward:
  driver = forwardfile
  file_transport = address_file
  pipe_transport = address_pipe
  reply_transport = address_reply
  no_verify
  check_ancestor
  check_local_user
  file = .forward
  modemask = 002
  filter
  suffix = +*
  suffix_optional

procmail:
  driver = localuser
  transport = procmail_pipe
  require_files =
${local_part}:+${home}:+${home}/.procmailrc:+/usr/bin/procmail
  no_verify

localuser:
  driver = localuser
  transport = local_delivery
  suffix = +*
  suffix_optional


thanks
Jeff