In relation to ventilation and damp/mould generally, bear in mind that the bathroom would have been built to the building standards that applied at the time. Presuming that the air vents and window met those requirements then it is arguable whether the OC has any further responsibility.
Air vents by themselves will not usually deal with condensation in a bathroom from a shower. The advice we have usually had is that you should open the window at least after the shower to let the condensation out. You don’t have to keep the window open at all times.
You could apply to the OC to install an exhaust fan – as noted above query whether they have to pay, as the condensation is coming from inside.
In terms of treating mould generally, don’t use bleach or borax etc., this will not kill the mould. The best treatment is oil of cloves diluted in hot water.