While you could argue this point (and probably plenty of others) with the building inspector it really isn't worth it.
The problem is, if everybody says "It's not worth arguing the point," the little dictators in building control get away with imposing conditions they have no authority to impose, and it only encourages them to try this trick even more.
The Approved Document for Part F is not law any more than the Approved Document for Part P is, but even so the comments you're referring to on page 18 clearly do not apply to all bathrooms.
First, for a bathroom with no openable window it stipulates a 15-minute overrun, but it's pretty meaningless since it doesn't give a reference point. 15 minutes from when? When the room is vacated? It doesn't say. And the first part of that section clearly states that intermittent mechanical extract may be operated manually, so I'd argue that the 15-minute overrun can, by those "rules," be obtained by simply switching the fan off 15 minutes after vacating the room - assuming that to be the intended reference point. But as this section says nothing about switching the fan in parallel with the light, how does the 15-minute overrun apply if you don't turn the fan on in the first place? Or indeed, if you
do decide to switch it with the light, the light is not going to be turned on every time you go in there anyway if there is natural light.
Second, even if there is no natural light in the bathroom at all, it merely states that the fan
could be controlled by operation of the room's main light switch, not that it
should be. If there is no natural light in the bathroom, then there can be no openable window either, so again, how is the 15-minute period supposed to apply?
But if the bathroom has an opening window, as many do, then clearly neither of these "requirements" come into play according to the Approved Document anyway.
That whole section is just a confusing mess which makes no sense.
