Ventilation

Subject – Ventilation & mould/mildew problems in bathroom

I am the owner of a flat involving a double brick exterior wall construction (early 1970s). In relation to the bathroom,  there are two terracotta wall vents on the outside and two vents on the inside. Four (4) vents in total located on either side of the bathroom window. The window is a sliding type, with a small mesh section inserted in the sliding pane. Each wall vent has a section of “flyscreen” attached/glued on to the back (from within the wall cavity). The flyscreen is blocked with dust for the inside vents, assume the same for the outside vents. The flat is located on the third level of the block. Some questions:
Q1 How do you remove the dust without dislodging the flyscreen?
Q2 Accept the flyscreen inserts are useless but realise there removal will restore proper ventilation to the bathroom. Therefore just push the flyscreen inserts off the vents, and if necessary reattach flyscreen (on the face of the vent). Note – I cannot access the external vents from outside the building, and cannot see them from the inside.
Q3 If these vents are not functional for air circulation, am I or the OC liable for the cost to install an exhaust fan if the wall vents are blocked and/or not adequate/functional for removing moist air (and the vents are recognised as common property)
Q4 Just keep the window open all seasons – even in cold/freezing winter
Q5 If mould/mildew appears keep treating it with the right product/s eg vinegar, bleach, borax

Hope Flat Chat can help

Thanks Max