Just another thought the idea of two intake grills. The ceiling in the bathroom will be lowered enough to fit some rectangular ducting so I could fit two grills one over the WC pan and over the shower. Is this gilding a lily or will this improve niff and vapour extraction? If viable would I be better reducing down from 125mm ducting to 100mm ducting to connect the two grills?

Finally which is better to fit a back flow restrictor immediately after the fan or use a flapped wall outlet? The distance from the fan position will be about 6m to the wall outlet.