[Exim] hitching post problems even with group = mail

I have this in my exim_mainlog:

2001-02-01 11:34:41 14KyNG-0000Hc-00 => /home/tim/in/egcs-list <tim@???> D=userforward T=address_file
2001-02-01 11:34:41 14KyNG-0000Hc-00 Completed
2001-02-01 11:34:41 14OEfV-0003bE-00 == /var/spool/mail/tim <tim@???> T=address_file defer (13): Permission denied: creating lock file hitching post /var/spool/mail/tim.lock.cyberelk.elk.co.uk.3a7949d1.00003a31 (euid=500 egid=500)

The FAQ says to put 'group = mail' in the local_delivery transport
configuration, but I have that:

local_delivery:
driver = appendfile
group = mail
mode = 0660
mode_fail_narrower = false
envelope_to_add = true
file = /var/spool/mail/${local_part}

Presumably though, I also need to adjust the address_file
configuration section to match. Is this correct, and if so should it
be mentioned in Q0201 in the FAQ?

Tim.
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