I'm doin a light for my mum at the bottom of the hall, its a 3 gang switch which does the downstairs light on one light, the outside light on another and the upstairs on the last, i have 4 red wires a blue an a yellow. Every combination i try gets 2 out of the 3 lights working and the other stays on constant.
OK, so let's try and get this right. The upstairs light would appear to be part of a 2-way circuit, as the presence of blue/yellow indicates there's a 3-core cable in there, probably linked to another switch on the landing. Be careful, as the number of wires present at the switch would suggest to me that the landing light is fed via the upstairs switch. If you have separate upstairs/downstairs lighting circuit, they will both need to be off before working on the 3 gang switch!!!
First, you need to deal with the upstairs light. Trace back the blue and yellow, you should find they come together into a single sheath which also contains a red wire and a bare copper CPC, which should hopefully be sheathed in green/yellow.
Usually you will find that yellow and blue are the strappers and red the common, but not always, so an educated guess says that you will need to pick one of the three switches and wire as follows:
Red - Common/C/~
Yellow - L1
Blue - L2
This then leaves three red wires left over, which will most likely be singles and each go back to their own sheathed cable, which will probably have an bare copper earth wire in there too, again this will hopefully be sheathed green/yellow and connected to the earth terminal on the switch. If the switch is plastic, the earths should all come together on the terminal in the backbox.
Of the remaining three reds, one will be a permanent live supply, and the other two will be the switches for the downstairs and outside light. In order to identify the live, you're going to need to turn off the supply to your lighting circuits (check for dead before touching anything and turn off the main 100A isolator for the entire house if in doubt!) and put these remaining three reds each into their own piece of unused terminal block. If you don't have any, pop down to your local DIY store and ask for some 5A terminal strip (sometimes known as 'choc block'). Turn the power back on, put one probe of your meter/voltage tester (NOT a neon screwdriver!) to earth, and the other probe onto each of the reds in turn. You will find one of them is live, keep a note of this and then turn off the power once again.
The red wire you have identified as live will need to go to Common/C/~ on BOTH of the remaining switches, so you will be needing another short length of red wire to link them both together. The two remaining L1 connections each need to go to their own red wire, it doesn't particularly matter which one. Do NOT link the L1 terminals together.
If you don't have a voltage tester of any form, I'm afraid you'll need to purchase one!
