  We present new polarisation observations at 13 cm, acquired using the
Westerbork Synthesis Radio Telescope (WSRT), of 65 sources, from the B3-VLA
sample of Compact Steep-Spectrum sources. These new data are combined with our
VLA polarisation data, at 3.6, 6 and, 21 cm, presented in a previous paper. Due
to the multi-channel frequency capabilities of the WSRT, these new 13 cm
observations enable a more reliable determination of integrated Rotation
Measures, and of depolarisation behaviour with wavelength. The new data are
inconsistent with the depolarisation models that we used earlier, and we
propose an alternative model which seems to work properly. We also revise our
previous model for the external Faraday screen, and its dependence on the
source redshift.
