After a lot of thinking and going back to a similar case, I am beginning to think you have your 4 blacks mixed up in the hall. But that mix-up relies on the red sleeving having fallen off the switch wire and inadvertently being placed on to the outgoing neutral wire.
In your hall rose, three of the black wires are neutrals and one is a switch wire. I think you have one neutral going to one side of the lamp holder (mistaken for and labelled as, the switched line), along with the red to the other hall rose and the other neutrals and the (real) switch wire are on the other?
At the hall rose, which I believe is the penultimate one on the circuit, the incoming neutral goes to the line side of the lamp holder, along with the red wire feeding the other hall rose.
The other neutral (which is going to the lounge, that is I believe the last light on the circuit) is connected to the switch wire (which has lost its sleeving, remember). These wires are connected to the neutral side of the lamp holder.
When the hall switch is closed but the lounge switch is open, the circuit completes as it would usually, illuminating the hall's lamps at full brightness.
When the lounge switch is closed, the circuit is completed through the lamps' filaments back to the neutral, giving a reduced brightness, as they are in series with each other, not parallel.
Is this possible?