My bulb is permanently on. Why?
To supplement what brhans says in his answer ...

Like this but without the middle cable (which is power to the next light &amp; switch, not present in last ceiling rose on a circuit).



You have old colours - black instead of blue, red instead of brown.

Your electrician used the correct two-red cable for the switch, most electrician nowadays use the "normal" cable and put tape around the wrong-coloured wire to indicate it's actual purpose.