Mould

I own an old miners cottage (circa 1900) and the mould problem is becoming unbearable. the humidity in the house is reading around 70% and we have condensation constantly on the windows (double glazed). We have tried to install vents in some of the walls, though we have probably not done it correctly. We've also tried insulating wallpaper on the outside walls, and I've now started resorting to adding mould preventative to paint. My neighbour said she had floor vents installed and she doesn't get any mould issues. Would this be likely to help? If so, what type of tradesperson would I need to install them? and is this likely to cost a fortune?