I am replacing new ceiling light in bathroom there is also extractor fan attached.there are 2 red wires which I have connected to live and 2 black I have connected 1 to neutral and the light works o.k. where does the other black wire go to make the fan work

Sounds like a switch loop. Where are the red wires, box or fixture? Where does the loose black connect to? It would seem one of the reds would go to the loose black. A VOM or multimeter would be a helpful tool to sort things out.

Suspect Grahamegolf is EU or UK.

Don't know wire colors but suspect red is live and black is neutral.

If that is correct you would connect both black to neutral. If you have single switch for fan/light wiring would be correct. If you have two switches then one red to the live of one switch and the other red to the live of second switch.

Just guessing here.

By Jove! Did the term "extractor" give you a hint?

No, it was the accent.