Mould in cupboard.

[Rampage]

We have a cupboard under our stairs that is against an outside wall. We seem to get a lot of mould growing on items in there, mainly shoes and bags. The walls / carpet and ceiling seem to be unaffected. There is no ventilation to the room. 
Would adding a vent help the problem? If so, vented to outside or through the door into the house would be best? 
We have also a mould problem in our bathroom which is on the other side of the wall to the cupboard, I fitted a timed extractor fan but this doesn't seem to have helped at all!
Finally I have noticed some blown plaster (behind wallpaper) above the cupboard on the stairs. Could this be connected? (picture below, hard to see but the plaster bulges out by around 5mm and is soft to the touch)

Thanks

 

[JohnD]

from what you say I will guess that the wall is damp due to a leak in the adjacent bathroom.

but it might be the floor.

vents for damp should go to the outside as the cool external air will be drier than inside the house.

 

[Rampage]

I forgot to say the bathroom is downstairs. The blown plaster is near the level of the ceiling of the bathroom.
The walls and floor are completely tiled so I don't think the water could get through that way? Also the floor is solid concrete not floorboards.

It's worth putting a vent to the outside do you think?

 

[JohnD]

might be a pipe buried in the wall

might be a leak above the ceiling.

I still suspect a leak.

You could leave the fan running 24hrs and see if that helps (running cost is negligible)

if the fan is noisy buy a new one.

 

[Rampage]

might be a pipe buried in the wall

might be a leak above the ceiling.

I still suspect a leak.

You could leave the fan running 24hrs and see if that helps (running cost is negligible)

if the fan is noisy buy a new one.

I think a leak is unlikely as the only water upstairs is for the heating cylinder and 2x header tanks, the pipe work for these is over 6 meters away.

I will try leaving the fan running constantly though and see what happens.

EDIT: just remembered there is a radiator above the bathroom, I will check this tonight thanks!