I am considering buying a 1930's flat but have seen obvious signs of damp - black mould on the wall, is this a problem?
It is a problem in that it indicates a high level of atmospheric moisture and an inadequate means of evacuating it, because black mould indicates a clear condensation problem.
Condensation is often down to occupancy lifestyle just as much as building construction issues such as insulation standards, heating provision and air circulation/extraction facilities.
For more information see the customer information sheet on our condensation page.
Building standards in the 1930's did not focus on insulation in the same way as we do today so you would need to check what property improvements have since been made in this area, as an upgrade will be necessary to take account of modern 21st century lifestyles.