Replacing downlight + extractor with standard downlight

[jayc89]

This is a plastering question, I haven't cracked, yet.

 

We have an LED downlight within an extractor vent above the bath. I want to replace that with a standard LED downlight in prep for fitting an MVHR extract valve instead. 

 

The ceiling is lath + plaster and in some rooms, it's more than 30mm deep, so cutting a section out to replace with PB isn't really an option (I don't think...).

 

What's the best way to fill the hole where the existing downlight + valve is to then be able to cut a smaller hole within the area that had been filled for the new, standard downlight?