Mildew/mould in loft

[Lanky]

We moved into our new build house a year ago and had the loft partially boarded. A couple of months ago I noticed mildew/mould (not sure which it is) forming. It was most noticeable on wicker furniture but was also on other items. It is getting worse.
Passing through the loft are a couple of flexible venting tubes that carry the the air from the bathroom extractor fans out through the roof. There also, what I guess to be, is the sealed top of the soil pipe.
Mildew/mould was not a problem in our previous house.
My best guess is that the venting of the air from the bathrooms is the cause of the problem, especially as the connections between fan and hose and hose and roof vent is just tape. Is there anything else that I should consider before I get the builders back?

 

[footprints]

Check the eaves are not blocked with insulation, the vents if sealed should not cause a problem but check around them or any down lighters for leakage of moisture into the loft.
Usually down to poor ventilation in the loft space.

 

[foxhole]

Agree with footprint .