You have wired the hall light to work in series with the outside somehow......
Describe what you have done.
Twin & earth from hall pendent to switch and then from switch to outside light. In the switch, the reds(browns) to the switch, black(blue) in a terminal block. At the pendent, live to the central loop terminal, and neutral to the neutral side??
Sounds like you have put the neutral on the switched live of the pendent. Maybe the pendent drop flex is wired back to front - fairly common to see this. The brown of the flex drop should be with the black core which is hopefully sleeved red.