A simplified model describing the polarisation characteristics of spun fibres is proposed, aiming at determining how close to a circularly birefringent medium such a fibre is. This is of crucial importance regarding the interest of such a medium for magneto-optic sensing using optical fibres, mostly exploited in fibre optics current sensors (FOCS). The spun fibre is modelled as a stack of thin linearly birefringent plates, each experiencing a slight incremental rotation. The eigenvalues and eigenvectors of the full stack of plates are determined, enabling us to assess how far a given spun fibre is from an ideal circularly birefringent medium for any spinning rate and linear birefringence. Experimental validation is carried out to compare the model results with real fibres.