S-PASS follows the IAU convention on Polarisation Angles (PA), as most of astrophysical data.
That means PA is 0 northward (of the coordinate system used) and increases eastward.
This is the IAU standard and if you use a standard astronomical FITS image you should be right.
However, we are aware that there are a few data sets and softwares that use a different convention, with PA increasing westward. Examples of popular data sets doing so are WMAP and Planck data, and the HEALPix software package. Passing from one system to the other means simply to change sign of Stokes U (or sign of the PA).
We suggest to bear this in mind when using S-PASS (and most of astrophysical data sets) in combination with WMAP and Planck data (and possibly other CMB data sets) or with the HEALPix package.