Top quark production in the process e+e- -> ttbar at a future linear electron positron collider with polarised beams is a powerful tool to determine the scale of new physics. The presented study assumes a centre-of-mass energy of 500 GeV and an integrated luminosity of 500 fb-1 equally shared between the incoming beam polarisations of (Pe+, Pe-) = (+/-0.8,-/+0.3). Events are selected in which the top pair decays semi-leptonically. The study comprises the cross sections, the forward-backward asymmetry and the slope of the helicity angle asymmetry. The vector, axial vector and tensorial CP conserving couplings are separately determined for the photon and the Z^0 component. The sensitivity to new physics would be dramatically improved with respect to that expected from the LHC for electroweak couplings.