1) Red wires (Brown in newer properties) rarely go to the light it's self. Some times twin red (brown) cores are used for switch but this is rare.
2) Following from above normally one of the blacks is a line wire not neutral and you must be careful not to get these mixed up.
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hi, look at the other end switch and note the colour of the core in the Com. terminal (prob Yellow).
put same colour in Com. of your new switch.
in L1 put 1 wire from each cable.
put remaining 2 wires in L2.
EG if yell. in Com 2xreds in L1
blue& black in L2.
As Ericmark says ALL wires should have red sleeving to show they are switch lines not neutrals.
all bare earths should be sleeved with green/yellow to show earths, which shouls be joined together.
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