  Some of the white dwarfs exhibit among the strongest magnetic fields in the
universe. Many of these degenerate magnetic stars are also rotating very
slowly. Among these objects, Grw+70$^\circ$\,8247, with its century-long
suspected rotation period and its 400\,MG magnetic field, stands as a
particularly interesting object. Surprisingly, for this star, the first white
dwarf in which a magnetic field was discovered, no spectropolarimetric
observations have been discussed in the literature in the last 40 years. Here
we present two sets of linear and circular polarisation spectra taken in 2015
and 2018, and we compare them with spectropolarimetric data obtained in the
1970s. Polarisation shows variability over a time interval of four decades, but
some subtle changes may have been detected even over a three year time
interval. Using the variation of the polarisation position angle as a proxy for
the rotation of the magnetic axis in the plane of the sky, we conclude that the
star's rotation period probably lies in the range of $10^2$ to $10^3$ years.
Our data analysis is accompanied by a description of our various calibrations
and tests of the ISIS instrument at the William Herschel Telescope that may be
of general interest for linear spectropolarimetric measurements. We also found
discrepancies in the sign of circular polarisation as reported in the
literature, and made explicit the definitions that we have adopted.
