clean it, paint over it and stick a nice joss stick in the room.
but seriously, i've had really bad mould problems in our first floor flat, all resolved by moving furniture away from the walls, cleaning, drying, then applying mould resistant paint on and adding a dehumidifier to each room for a few hours a couple of days a week.
you can investigate till your broke and broken and still not completely eradicate it.
my bathroom is a pain in the arse. cant even apply paint to one of the walls for it flaking off about a week later (think the problem is that an old bath drain outlet pipe is still attached to the outside wall - creating a 'cold air bridge'(sic) so the moisture comes right through and peels paint away.
ive given up trying to figure out another way of solving it, without having to get scaffolding in (its on a really awkward part of the outside wall right above the below flats massive conservatory) so i just sand it back and paint over it again every now and then.
air flow is the key thing here. keeping your place as dry as you can, and treat it with some lovely chemical paint.
the back of our drawers was green once, when i went to wipe it off, spores went everywhere - wiping with a damp cloth prevented it kicking up so much, so be careful when removing mould, although ive read its the black stuff thats the nasty stuff, but dont quote me on it.
we have a mixture of cavity walls and solid brick walls. sod getting stuff done to the outside - all sorts of bullshit costs and permissons are probably needed. so do the cheapest and easiest methods first. if they work - or make it liveable, then why bother doing anything else? life is too short. just spend more time out on your bikes