Hi ASH, sounds like you might need a new dimmer!!!
I am not sure why you have such a combination of wires, I think maybe the black unused may in fact have pulled out of original sw. when you removed it?
If 2 twin/Earth cables then would have 2 reds + 2 blacks,
Is it a 2 way sw. across the room to another or ......
have a close look at colours also see how many wires / colours @ light,
may be L + N in to box & sw. L + looped N to light
more info please SPARX
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