Cannot deliver local mail to root

I'm running exim 1.73 under Red Hat Linux 4.2 on a Sparc 5. Mail delivery
to local users works fine except to root which causes the following error
message:

(13): Permission denied: creating lock file hitching post
/var/spool/mail/root.lock.aire.open.ac.uk.3441cc57.00001ad6

(for example).

/var/spool/mail is writeable by user mail:

drwxrwxr-x   2 root     mail         1024 Oct 13 08:24 .

and I have the following for local delivery in the configuration file:

local_delivery:
driver = appendfile;
file = /var/spool/mail/${local_part},
group = mail,
mode = 0660

Can anyone spot what I've missed?

Thanks in advance,

Mike
-- 
Mike Zanker                | Email: A.M.Zanker@???
ACS Network Services Group | Tel  : +44 1908 652726
The Open University        | Fax  : +44 1908 652193
Milton Keynes, UK          | PGP public key available

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