That's good. 
On looking closer, I mistook the wiring at the downstairs switch - it is as it should be. The two circuits are not connected.
However, the fact that you had this problem indicates that you probably have a loose or wrong connection somewhere.
In the loft switch - Red must go to COM but Blue and Yellow either way round.
Also, at the intermediate switch, although it makes no difference, the same colours are normally put on the same side of the switch.
On looking closer, I mistook the wiring at the downstairs switch - it is as it should be. The two circuits are not connected.
However, the fact that you had this problem indicates that you probably have a loose or wrong connection somewhere.
In the loft switch - Red must go to COM but Blue and Yellow either way round.
Also, at the intermediate switch, although it makes no difference, the same colours are normally put on the same side of the switch.