The existing ceiling mounted fan is not working correctly now. It is wired via an isolating switch high up in the room away from the shower. The fan has a built-in pull cord: pull for on, pull again for off. There is no timer. I just want to replace it with a similar model. The electrician advises that I must fit a low-voltage model with an isolating switch and transformer outside of the shower room, in the bedroom next door. The existing fan was installed when the house was built 18 years ago. Any advice please?
The replacement of a device (or any part of a circuit) will likely be subject to all current codes, laws, and rules (may not be applicable to cord-and-plug devices). What these "rules" are will depend on your location, so it's difficult to say if it's legal or not without knowing what that location is.
Your best bet would be to contact your local government, and ask what the "rules" are.