I recently tried to fit a metal light switch and have tried all combinations and can get the light to work but when its on it turns off 3 other lights upstairs?
I have gone back to putting the origanal plastic switch back on but again have had the same problem, it worked fine before, all 3 wires are brown but I have marked them and whatever way round I put them I either have no lights work, or one will work and 3 don't and vice versa... its probably so simple but I need help!

can get the light to work but when its on it turns off 3 other lights upstairs?
OK, wire the switch up that way. You should have one wire connected to one terminal of the switch and two wires connected to the other terminal.

Take one of the two wires connected to the one terminal and connect it to the other terminal. If you chose correctly the switch will turn the light on and off and the lights upstairs will stay on. If you chose incorrectly the switch will turn the lights upstairs on and off and the light will stay on. If that happens, put the wire you just moved back, and move the other wire to that terminal.

You should not that you are not in the US or Canada (UK I assume).