The authors report the results of measurements of transverse polarisation normal to the reaction plane of K-shell photoelectrons produced in a gold target by unpolarised 662 keV gamma rays. Energy selection of the photoelectrons was performed using a double focusing 90 degrees sector magnetic spectrometer specially constructed for the purpose. The electrons were counted in 300 mm² silicon surface barrier detectors of 1 mm depletion depth and the polarisation was deduced from the left-right asymmetry observed at backward angles in Mott scattering from a gold target. Measurements were performed at six values of the mean angle theta between electron and photon momenta with the results: theta =0 degrees , C₀₂=0.009+or-0.017; theta =6 degrees , C₀₂=0.173+or-0.018; theta =10 degrees , C₀₂=0.160+or-0.014; theta =20 degrees , C₀₂=0.133+or-0.025; theta =30 degrees , C₀₂=0.162+or-0.023; theta =50 degrees , C₀₂=0.198+or-0.064.