It seems that after running "slapd_db_checkpoint -1" and placing a checkpoint I can then run "slapd_db_archive -a" and get returned all the redundant log files - hooray !
Quite how these used to get returned (and they did) without a checkpoint directive either in slapd.conf or DB_CONFIG I'm not sure.
I plan to add a "txn_checkpoint" entry to DB_CONFIG immediately :-)
Thanks.