Just for reference I've resolved the issue. Shot meself in the foot I did. The project is in 2 pieces: an executable (checks for and copies an updated app then executes it) and the app itself. The VCX was accidentally being compiled into the exe so that when the app included new classdefs the runtime couldn't locate them in the exe library. Probabily a newbie style error and, therefore, embarrasing. Oh well.....