Ventilation for Internal Bathroom in Flat

[Leofric]

An accessible ceiling / roof void of 65 millimetres or greater, to accomodate the flat ducting? Was this a serious question?

Yes, you could have been talking about the dimension of the ducting itself which you can get in 234 or 308mm wide by 29mm depth ,but I understand your point that the ceiling void needs to be bigger than that.
ps keep your hair on

 

[Gizasmum]

Many thanks to both of you for your help