I put the engine in the frame to provide more leverage for doing up the primary nut and then had that light bulb moment, my torque wrench isn’t bi-directional, so only torques right-hand threads.
Quick Amazon purchase and I now have a bi-directional torque wrench, bizarrely the ratchet head pushes through, which means its still only torqueing right-hand threads but as its now reversed its a left-hand thread, who knew!