A fix for this problem has been found.  Although slightly different to the one we experienced, the problem is described at :

http://support.microsoft.com/default.aspx?scid=kb;en-us;814113

...and the hotfix for it can be downloaded at : http://www.microsoft.com/downloads/details.aspx?familyid=9814AE9D-BD44-40C5-ADD3-B8C99618E68D&displaylang=en

Hope this helps anyone else out there who has experienced the same issues with the DTS GUI.

(With thanks to Paul Hiom at RHI/CSC in Birmingham for help with this).