Thanks Mark that appears to be the most likely problem, others at our firm have 5.1.6.747 and their installs are working fine and I beleive my old client was Oracle 9i.
I have a new pc installation and have had pl/sql developer installed. However when i go to logon i no longer have my list of DB's in the database dropdown.
Assuming that this is read from the tnsnames.ora file i copied my old copy into c:\oracle\network\admin\tnsnames.ora however pl\sql still...