replacing two ceiling lights and a double dimmer

[Pricey]

Here's the scenario folks -
The bread knife has persuaded me to replace the two single bulb ceiling lights with two, three bulb ones. She has also requested replacement of the double switch with a double dimmer!

So far I have replaced the switch with the dimmer and managed to get one of the lights to come on and off and dim correctly, however, the other one either remains on permenantly (suggesting I've by-passed the switch, I think) or doesn't work at all.

It's a newish house and the builders weren't fussy about the core of cable they used for the wiring. In the light that won't work I have two cables coming from the ceiling - 1 a 3 core and 1 a 4 core. The 3 core has, earth, black and red and the 4 core has earth, blue, red and yellow. I am assuming the yellow is the switch feed as there is also a yellow one in the switch. I've tried all sorts of combinations but thought that this one would have worked - both reds (one from each cable, into the block and out through the 3 brown of the light fitting, back through the 3 blues of the fitting and out through the yellow to the switch. The blue from the 4 core and black from the 3 are isolated into a separate block.

The switch has the yellow into L1, 2 reds in Com, a red from the second light in the other L1 and another red in Com, with a small bit of wire joining the two Com's. The switch also has the blck from the working light, another black and a blue into a block isolated in the switch.

As I say nothing happens in the main light but the other comes on and dims ok.

Any ideas before I rip the lights down and shove them where the sun don't shine!!

cheers
Pricey

 

[MANDATE]

firstly the 4 core is widely used between a couple of two way switches and is not required at a ceiling rose, if this is being used then one of wires in the cable will be redundant.
I would suggest the 'red' in the 3 core will be the incoming live, you could prove this if you have a mains tester, this would go to middle terminal block in a ceiling rose.
Now if I was to use 4 core I would use the 'red' to run from the middle block down to 'com' on the switch (showing its live).
I would choose yellow to run from L1 on the switch back up to the rose, this yellow would have red sleeving on both ends to indicate it's 'switched live'. At the rose it would go to terminal 1 from where the brown wire would go to the lamp.
The blue wire from the lamp would go to terminal 3 and so would the black wire from the 3 core cable.
This leaves the blue wire which is not needed and should be insulated at both ends in seperate terminal block or cut off so it can't be used.
One of the problems is when someone sees a yellow wire or a blue wire at a switch they will think they are dealing with two way system and will cause some confusion
Most ceiling roses have three cables ie incoming cable / switch cable / outgoing cable to next rose, all except the very last rose which only has the two cables. (I guess the builder ran out of 3 core cable)
If youre not clear on the functioning I suggest you draw it on paper first

 

[Pricey]

Thanks for the advice regarding the two lights and double dimmer switch. It was only when I bought a mains tester that I finally realised the dimmer switch was faulty and only worked from one block!!

Changed the switch and it works.

cheers