[exim] Problem restoring messages to the queue

An over-enthusiastic anti-spam system moved a lot of messages from a
split queue into a quarantine directory. Having identified and
removed the actual spam (data and header files), I tried to restore
the remaining messages to the queue by moving them (-H and -D files)
back to one of the split queue directories (/var/spool/exim/input/q,
picked at random). However although the queue count returned by exim
-bpc rose by the expected number of messages, Exim then complained
that the spool data (-D) files were missing, even though I can see
them in the directory alongside the -H ones.

Googling for information on moving messages in and out of the queue
suggests it usually is just a case of placing the files in a queue
directory, however this isn't working in this case. The examples
related to moving queued messages from one Exim server to another,
rather than in and out of the local queue, however.

I'm guessing that because the messages were originally from the same
queue, Exim has a record somewhere of which of the split directories
they should be in, and is looking for them there instead of where they
now are?

I found a reference to removing the files in /var/spool/exim/db, which
are apparently auto-rebuilt, but there wasn't any surrounding
information and even on a dev box I'm reluctant to blindly follow
random instructions from unknown web pages.

Is there a recommended procedure for restoring messages from an
off-queue location to the queue?

Meanwhile the anti-spam system has been modified to freeze suspect
messages for later inspection instead of trying to move them out of
the queue.