New home leak

Nevermoving

Hi all, looking for some advice on what this possible leak could be? We moved into this property on Friday, survery did not flag anything about possible leak/damp. We noticed the corner of the room had a leak stain, which continued to get worse throughout the week. We got a roofer over to take a look and the roof is completely fine (one cracked tile at the front of the property that could not be causing the leak). The wall is in our bedroom and adjoins to the neighbours wall. We have spoken to the neighbour but they've had no issues. We are running a 20L dehumidifier, which has helped a lot (it was half full over night and then again today). We are not sure who to call out next to take a look at it? Any help would be much appreciated. 

Nevermoving

These pics are before we used the dehumidifier 

Alter_ego

Tank in loft?

Nevermoving

There is a tank in the loft, but couldn't see anything leaking. Maybe it needs relooking at...

FreeBear

External corner ?

Have you checked that the gutters/downpipes are not blocked ?

Nevermoving

Yes, the roofer cleared the guttering but unfortunately it hasn't helped. 

daveyjp

Is the loft insulation tucked all the way into the edges of the roof?  If so loft space condensation could be the problem.