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[VirginiaP]

I have a 2 gang light switch in my hall where 1 switch controls an outside light and the other switch controls my hall light along with a 1 gang switch at the opposite end of my hall (2 way switching). The cables in the 2 gang switch are 1 red and 1 black wire then a second cable which has a red, a blue and a yellow wire. The 1 gang light switch has 1 cable with a red, a blue and a yellow wire.

I previously had brass switches but because there is no earth presence decided to change these to non metal, I did make notes of how these were wired before I removed the old switches but obviously did so incorrectly as I am now unable to get these working correctly. If I get the 2 way working correctly in the hall then the outside light won't switch on and if I get the outside light working then the 2 way in the hall doesn't function correctly.

The terminals in the 2 gang are Com L1 and L2 and Com2 L21 and L22

Any help would be greatly appreciated.

[Mr White]

Despite what you (or anyone) would assume there is no hard and fast rule as to how a switch is wired up (As in what colour does what or where that colour wire should go)
Without being there to see all anyone can do is guess.
My guess is that although you believe you knew where all the wires were, I suspect that there was a small "link wire" (Which would have now long vanished) and this link wire was, in fact, acting as a common live.

But as I said, without being there to physically check, all I can say is you should get a competent person in to do the job for you. It does NOT have to be an electrician.