Adding a security light to my garage

I want to add a security light to the outside of my garage. I have a single light switch which I would like to change to a double switch- one switch for the security light and the other to remain for the inside garage light. How do I wire the new double switch? i have managed to feed a lighting cable identical to the ones already there down the conduit to the switch. At the moment 2 cables go to the switch and can be traced back to a main box with trip switches. cable 1 has one red wire connected to a COM point (i presume live) while the neutral is linked to the neutral of the other cable via a little block. the earth wires of the 2 cables are aso linked together at the little block. the red wire of the 2nd cable is attached to a L1 point on the switch. My cable is at present taped up and not being used.

Get new double switch.

Remove the two wires from the existing switch and connect to the 1st new switch in the same way (Com and L1)

Your new cable connects like this:
Live / brown - to L1 on the 2nd switch
Neutral / blue - to the existing block where the other neutrals are
Earth / greenyellow - to the existing earth block.
The other end connects to the new light.

Now link the two COM terminals on the switches with a short length of brown wire. (One COM terminal will also have the wire for the existing light in it).

If you find that the new light only works when the existing one is on, swap the wires in COM and L1 on the first switch.

many thanks for your help.