I live in an old flat on the 4th floor of a housing block. The bathroom extractor fan is old and weak, and there is some distance between the bathroom and the outside, which means the bathroom often gets damp: here is a diagram

I would like to replace the extractor fan, however, the current extractor fan is quite difficult to access, with only a small port to the fan in the side of the ducting. I also can't see any obvious fixings which attach the old extractor fan, so am not sure how I would remove it.

If I am not able to remove the extractor fan, would fitting a second extractor fan at some point in the ducting be sensible, and are there any considerations I should be aware of before attempting this?