wiring a bathroom fan

[plugy]

Any ideas/help.
We are renovating the bathroom where all wiring is in the loft and over boarded so cannot get at.
The lighting circuit is wired via a junction box somewhere under the floor. So only exposed wire is the live/neutral for the pull switch and at the other end, live/neutral of the light. We want to add a fan with an isolator switch and fan. What's the best way of approaching without having to pull up the floor upstairs or drop the ceiling? Any little drawing to help would be appreciated

 

[EFLImpudence]

At the switch will be a Live and Switched Live.
Live and Neutral at the light.

Therefore you will have to run a cable from the light to the fan but you will only be able, with what you have, to have the fan being on and off with the light.
A switch or isolator would only allow switching off the fan when the light is on.

For any arrangement with more options, you will have to access the junction box.

 

[plugy]

Though as much, appreciate the help