Installation

[misto]

I want to install a Porch Lamp connected to a single pole switch via a 2 way - 3 gang switch. In the single pole switch, I have wired in the two red wires to the terminals and joined together the 2 blue wires. In the 2 way switch, there is 1 cable for the hall light and 1 cable for the landing light.There is also another cable which is 4 core - Black/Green/Red and Yellow. To which of the terminals do I connect the single pole switch to ?

[AlBags]

Look for a common live .. (one that remains live regardless of which position the hall and landing lights are in) You had better hope there IS one. It's not unheard of to have no common available at one switch in a 2-way system (the common will be at the other end)
What does the red do? Check that first.