Potential damp issue - advice needed

Hi,

A bit of background on bullet points for ease of explanation:

• My property is a 1950's bungalow.
• The room in question has Marley thermoplastic tiles (yes, they do contain asbestos) stuck down with bitumen adhesive.
• the condition of the exterior walls is as you'd expect from a property that was last rendered probably about 20-30 years ago! There are some small cracks, some flaked paint and a little bit of repointing on some brickwork on the bottom part of the wall that could be done to improve it.
• I had the room re-skimmed back in December with the exception of 1 wall where the window is due to a leak.
• The leak was coming in through the ridge above the window (gable end) down the cavity and out from above the window.
• Roof was replaced in late Feb and the final wall was re-plastered at the beginning of March.
• After 7 days to allow the plaster to dry, I have poured Wickes SetCrete High Performance levelling compound over the tiles (used their acrylic primer first, un-diluted, as per the advice from manufacturer)

Here's where issues start to appear:
When the first lot of plastering was done, I noticed that a corner of the room (exterior walls) took a lot longer than the rest of the room. I could visibly see that the floor was moist but with some good ventilation and warmth from the nearby radiator it did eventually dry.

When laying the levelling compound, I noticed that the primer took longest to dry in that particular corner as well. I should note that I made sure the floor was clean and as dust free as I could get it before priming. However, the levelling compound did dry fairly evenly so didn't think too much of it.

I have now in the last few days come to paint the room. I have used a 4:1 diluted leyland contract matt (that mix ratio suggested by a decorator friend). The next day I notice that there is section of darker wall in the corner and the floor is visibly moist as I can see a darker shade of grey compared to the lighter shade of grey which the compound has dried in.

I applied a second mist coat last night but drew a line across the corner on the floor using a sharpie as I wanted to see overnight how much the line expands or retracts. It has grown by about 2 inches further out from the corner.

It should be noted that there was no heat or ventilation in the room until this morning when I took the plastic sheeting off the windows (I have been using a paint sprayer). The radiator is still off the wall so no heat in there.

From what I have described, does my issue sound like it could be a penetrative, condensation or rising damp issue? And can anyone give me a few tips to test to try and resolve the issue before bringing in costly contractors.