Ventilation issue

Maintenance

I have a new property with timber single glazed windows to the front of the property (it's in a heritage area and windows are over 100 years old). The new tenants have noted that the condensation is building up in front bedroom windows overnight.

This is causing the mildew on the blinds and curtains. When the windows are closed, there is no other ventilation to the room - this has been particularly bad over winter.

In dealing with this, I'm debating on secondary glazing/new double glazing to the existing heritage windows, but concerned that this might not deal with the there a legal requirement that these rooms should be adequately ventilated (i.e. have some form of grill/trickle vent to the outside) and as a rental property is secondary glazing a legal requirement?

Thanks in advance,

John