To be honest you would be better off getting a proper single gang two way switch. However...Can I just check I understand correctly?
L2 red and L2 black
All 3 wires (red, blue and yellow) together into top L1?
L2 red and L2 black - from 2 core sheath - earth to back box.
Then you need to record which colour cable goes into which terminal at the OTHER hallway switch - E.g. Common = red L1= black and L2 = yellow then at the new switch put the Common colour into L1 left. L1(other switch) into L2 (the one with the red cable) and L2(other switch) to L2 (the one with the black cable)