DIY Doctor UK DIY Forums

I am replacing my old pull switch to a dimming pull switch.
There is a black and a red wire and also a seperate red wire in the old unit, why 2 reds and one black, there is a red and black coming out of one cable with seperate red.
any help? :?:

Hi if this is a bathroom/wc is there a fan also?
If so it prob. will have a L, SW - L & N more info pls.