Lockfile / Write error whilst testing

Hello all,

I am trying to recover from a catastrophic system failure in which I
also lost my backups (don't ask!).

Before I attempt to bring my mail-chain back on line I wanted to
recreate my procmail rules, and before I did that i wanted to test them.
I have used Sean B. Straw's sandbox, and have as yet no /etc/procmailrc
or even a $HOME/procmailrc. All I have is the sandbox.rc file and the
recipe I am currently trying to test (the one that sorts my many mailing
lists).

I have fallen at the first hurdle. The recipe correctly identifies the
correct mailing list, but then fails to write it to the mbox file in the
testing area:

=================8<============================================================
procmail: Match on "^List-Id:.*mod-security-users"
procmail: Locking "MLists/Mod-Security.lock"
procmail: Error while writing to "MLists/__g.YRNOQB.mydomain.org"
procmail: Terminating prematurely whilst waiting for lockfile ""
From [email protected]  Sat Aug 25 14:34:16 2012
  Folder: **Bounced**							      0
=================8<============================================================

This is the relevant part of the recipe:
=================8<============================================================
:0:
* ^List-Id:.*mod-security-users
MLists/Mod-Security
=================8<============================================================

Finally this is the sandbox.rc (stripped of most of the comments)
=================8<============================================================
# Sean's little sandbox.
#
# 20020619/0915 SBS		Updated with additional notes, added standard header
#						extractions and defined ORGMAIL.
# 20030424/2019 SBS		more header extractions
# 20040511/1212 SBS		Added RELAYHOST extraction
# 20041212/1124 SBS		Revised RELAYHOST to include original HOST
#
{snip}
MAILDIR=$HOME/Proctest
COMSAT=no
NL="
"
wsstar='[ 	]*'
SANDBOX=1
LOGFILE=OutFiles/testing.log
TEMP=/tmp
VERBOSE=ON
LOGABSTRACT=ALL
DEFAULT=/home/mark/Proctest/OutFiles
ORGMAIL=/dev/null
SENDMAIL=./sendmail.sh
AUTOREPLY=$HOME/.procmail/autoreply
# NOTE: I MAKE EXTENSIVE USE OF SOME OF THESE VARIABLES WITHIN MY OWN
# RECIPES.
{included by snipped here...}
INCLUDERC=Filters/rc.mlists
=================8<============================================================

In desperation I have even tried making the permissions 777 on the
$HOME/Proctest/OutFiles/Mlists directory. Still fails.

What have I messed up?

Thanks in advance

Mark