Mold :(

When we owned our flat, we had a similar issue with mold - we bleached the heck out of it, got humidifiers and that seemed to keep it at bay (as well as anti-mold paint).  The windows needed to be regularly cleaned with bleach (probably once a month) to keep them from re-molding.  Best of luck!

Aside from Bleach, you can use Dettol Mold & Mildew remover.

As an aside, be careful with that stuff. It works really well, but I'm 90% certain that it caused our cat to have a seizure. Just under a year ago I'd done the shower with that product and while I was rinsing the phone rang. My hunch is that the cat got it on his paws (he's a water baby) while I was on the phone. He's never had another seizure and the vet suggested that if it didn't happen again the most likely cause was some sort of toxin.... 

I also just realized that I left something really relevant out!  This cabinet is situated directly about the radiator controller thing.  The hot water heater?  You can see in this picture what I mean: it's in the cabinet with the folding-type door
http://farm4.static.flickr.com/3201/3108094038_938602337a.jpg?v=0

I think you have your answer right there.  What's probably happening is a bit of warmth from the hot water tank is seeping out and then condensing as soon as it's a bit cooler.  In any case, that is awful and the landlord should never have rented it, or at the very least let you know about the mould before you moved in.  That's a seriously nasty patch.

For the strip along the caulking, we just started using Parozone Grot Buster on our windows in the bathroom.  It's a really thick concentrate you just slap on the caulking, leave it about 10 minutes and it literally erases it.

I'd get in touch with the landlord ASAP about the mould and tell him to get it sorted before you contact the council regarding tenants' rights.

Hopefully your landlord will be better about this than ours was - a threat to call Environmental Health was the only thing that lit a fire under him.  We knew there was a leak in the roof that was causing seepage that was causing mould high on a wall - convincing him of that took a while, and then took even longer to fix. 

While waiting for the roof to be replaced (summer of '07, it was SO wet and rainy!), the landlord provided the de-humidifier and also had an airbrick installed into the room to help circulation - it didn't really make a difference, since it was summertime and the windows were always open.

We took photos of everything as well, hoping to be able to document things if necessary.  Happily that didn't have to happen - hope your problem gets fixed - that looks much worse than what we had to deal with!