Many '30s buildings have condensation-related mold problems. This is often related to double glazing being fitted without trickle vents. If you do buy, the first thing you should do is see about retro-fitting trickle vents to all the windows in the flat. I got mine done, and at around 70 quid per vent, it's not that expensive. It also helps if your chimney isn't blocked up as that acts as a major ventilator in the living room. You'll be surprised at how much this might do the trick with the mould.

If this doesn't work, there should be no problem in fitting ventilation, as long as you do it in connection with the managing agent, as technically the outside walls aren't yours to mess around with!