I think Shedman is right Vicky............. mould from condensation rather than damp. I guess the house is quite old. Cold walls with no insulation I expect. Vapour from cooking, washing, breathing etc condenses on the coldest spots. Heating and ventilation are the only remedies with cold masonry.
If you're renting then Shedman's ideas to mask the problem sound a good way to go. If you own the flat, then It's a straightforward job to add insulation , and, replaster the walls to beat the problem for good.