The wall is cold in the winter even with the cavity.
The air in the kitchen is warm and carries lots of moisture from cooking and boiling the kettle.
Condensation is forming on the wall and generally you may not see it to wipe it away so over time mould will form.
There are several solutions that will help. First of all scrub the wall with dilute bleach and let it dry off as much as possible - move furniture out of the way - get a fan playing over it.
When you cook try and have a window open to let the steamy air out. Keep the kitchen a little cooler if you can and ventilate the kitchen as much and as often as you can.
If you can get hold of a dehumidifier and run it in the kitchen it will help quite a bit.
Doing these things will help but it may be a continual problem for you in the winter depending on the coldness of the wall - lack of ventilation and the naturally steamy air in the kitchen.
Good luck...
Peccavi, February 2009
