My hall light has two switches for hall and landing however when I turned the hall light on earlier it turned off the lounge light. On turning the lounge light back on the landing light also came on. Why has this suddenly happened? I now have my lounge ligts on but have not touched hall light since. I know this problem obviously needs sorting but is it cause for immediate concern? Thank you

If there has been no electrical work going on recently to cause these irregularities, you may have some arcing or possibility of arcing going on with these switches or possibly a junction box or fixture. Since arcing is a major cause of home fires, it could be a matter of immediate concern. Has anyone worked on any of your wiring recently?

Assuming that these are single pole switches, meaning that one switch controls the hall light and the other controls the landing light and another switch located somewhere else controls the lounge light.

Remove the switch cover plate.
Remove the switches from the box.
I am confidant that you will find that the switches have been wired using the quick connect holes in the back of the switches.
Use of the quick connect holes is known to develop bad connections over a period of time.
Remove the wires and put them under the screw terminals on the side.

Removing the wire from the quick connect holes can be annoying at times. Sometimes you can just pull while twisting the wire back and forth and get them out. If that doesn't work you have to use a sharp pointed tool in the release hole next to it. Another piece of wire usually works for this. If you can't get the wires out, just clip them off as close as you can to the back of the switch.
What you will probably find is a wire (black) connected to one switch, a short jumper wire next to it, going to the other switch. At the other switch will be another black wire. One of those wires is the power coming to the switches. The other is the power going to the lounge light. The short jumper wire of course connects the two. This can be at the bottom of the switches or at the top.
At the other (top or bottom) will be a another black wire on each switch. One goes to the hall light and one goes to the landing light.

Remove the jumper wire and the two wires next to it. Make another jumper wire. Twist all four of these together and put a wire nut on them. Connect the two jumper wires to the switches using the screw terminals. Wires must go under the screws the same direction as the screw turns to tighten.

Connect the other two wires to the other screw terminals of the switches.