In the drawings: [0042]FIG. 1 is a schematic representation of an embodiment of an assembly according to the present invention, the assembly being positioned in a wellbore; [0043]FIG. 2 is a view showing the orientation of FIGS. 2a and 2 b. [0044]FIGS. 2a and 2 b are a longitudinal section along a casing section for a deviated wellbore junction useful in the present invention; [0045]FIG. 3A is a v