So many things to think about, Rosy.
A house ay that time would have had a fairly basic damp-proof course installed ... probably either a coat of pitch or a course of slate applied to the brick bed.
These are known to fail over time. Any rising damp would be held back from entering the room by the tiling. It's not unknown, in these circumstances, for it to rise and show above the tiled area.
No tiles, I guess, in the storage area, but, if it's a cupboard, then lack of ventilation could result in condensation/mould deposits on surface. Kitchens generate a lot of water vapour. Do you have good extraction? Do you have gas cooking?
The first, and simplest thing is to make sure the chimney flue is well ventilated top and bottom. Check this first. It may well be the cause here.
If it is a DPC-related problem, then I would suggest some remedial works involving some re-plastering, but look at the flue ventilation first.