Why do my Mondeo courtesy lights not work when doors open?

On my Mondeo [2l Ghia S reg] the interior front and rear courtesy roof lights do not illuminate when their switches are set to the 'door' position and any of the doors open.
However, the footwell lights do always come on, which indicates that the door swithces work. The courtesy lights do come on when the switches are set to the 'On' which indicates that the bulbs work.
The Haynes manual circuit diagram shows that the footwell lights and courtesy roof lights connect to the same point on the fusebox, which suggests that the fusebox is not the problem.
Any ideas what could be wrong?