Update

Things have been going slow, I've not had much time with my lectures and studying, and the paint took forever to dry.

Pictures speak a thousand words so I'll try and stick to pictures.

First, we've painted everything white as a primer to see how the freshly rendered walls would react to it.
This first layer took almost a week to dry, due to problems with humidity and temperature in the room. (And a granny.. but more on that later)

The plasterboard ceiling got painted three times in total, the walls once. 

We decided on having a dropped ceiling line (apparently its a traditional thing) so the top 15cm of the wall remains white. I carefully went round with masking tape and a measuring tape and taped off 15CM all around the room

Next, the wall paint went on, it's called "Egyptian Gold" or something... Dulux colour.
This took two layers, and also took over a week between coats. 

Once all that was out of the way, I did the "final" screed layer for the floor, all by my self, and as a result the concrete screed was starting to go off (Warm room!) fairly rapidly so I had to rush it a bit.

The concrete floor underneath wasn't level at all, despite all the complicated levelling with pipes and god knows what which took over an hour to do (Not my idea..)
As a result in one place it was 3cm thick, the other only 5mm.
Anyhow, all more or less levelled out, left at like 8:30 at night after granny was chasing us out because she wanted to go to sleep, made a right mess of all the material due to lack of light to see what we were cleaning outside (It was also freezing outside).

Next day I get called by my girlfriend who comes to present pictures and tells me the whole floor is horrible, its squint and has dips and rises it in EVERYWHERE.. I think to myself what the hell... It can't be..

She presents me this picture of her granny holding a piece of wood on the floor

Worried I go to inspect it myself, and the floor isn't quite level, there are some dips that I knew of between the runners (I did it in three stages) which would require scraping back down with a plane, but its soft concrete so its easy to do.
I checked it with the plank and right enough, huge dips and bumps of over a centimeter, I grabbed the spirit level and suddenly the dips and bumps were only 3-4mm, at each point where an old load of screed met a new one. The plank was crooked!!!

We went out and purchased some self levelling screed (Baumit quatro nivello or something) just to level the last edges off.

I cut away all the excess damp proof membrane and bits of perimeter insulation, chipped away all the render that had fallen between the wall and the perimeter insulation, filled up the big gaps with some concrete, left it all to dry for a bit and then started with the self levelling screed.

Fun fact, I looked up the product data sheet on internet in English, and it stated 7.5L water per 25KG.
The instructions on the back in Slovak stated 6L per 25KG...  I figured I better follow the instructions on the pack, and the first batch I made wouldn't flow!!! It just sat there in a big heap, if you floated it around to level it out you would leave streaks in it which wouldn't settle back down, almost like a cake mix.

I managed to somewhat level it out and wasn't impressed.

The next two batches I mixed with more water (7L) and it flowed a lot better, I even managed to fix some of the original mess with the too dry mixture.

Anyhow, so it now looks like this:

Left that to dry for a day, and moved onto the electrics.

fitted all the face plates and connected them up, but they aren't connected to anything upstairs yet, the ring is complete but its not hooked up.

Now, the damp I mentioned earlier...

Basically the new wall's corners and the ceiling where it meets the wall keeps on forming condensation.

It is very hard to explain to a 75 year old how damp and condensation works, especially if you don't speak the same language.

Granny insists on keeping everything shut, with the heating turned up to 25C.

The outside walls are insulated with polystyrene, the kitchen ceiling and 3 out of 4 walls are insulated with polystyrene. Damp can not pass through it.
She doesn't use the extractor fan as its not connected to a flue properly.
We poured almost 2000L of concrete, painted on 25L+ of paint, used around 80KG of render on the walls. All of this contains water, that water (when heated up) evaporates and needs to go somewhere.

Add to that drying clothes, boiling water in the kitchen, doing dishes. All that moisture needs to go somewhere.

It can't go out of the kitchen as its a air tight box. The only place it can get out is into the bathroom, into the hall (Which it won't because the door is ALWAYS shut and has foam around the door to stop air passing through it) or the new room we are working on.

The bathroom has 2 out of 4 walls insulated with polystyrene, leaving it with the option to go through the ceiling, back to the kitchen, or into the new room.

As a result, with the heating turned on in the new room, all this moisture becomes active and wants to go somewhere, it rises up until it meets a place that is cold enough to condensate, which happens to be the only breathable wall in the whole house.

Granny refuses to open the window to let the moisture get out with the heating on, so either the heating is on with window shut (Moisture condensates in cold corners) or the heating is OFF with the window OPEN (Which cools down the floor slab and walls, and the moisture isn't active enough as its too cold.)

It would help if the moisture could escape through the plasterboard ceiling and up and out, but she refused to switch on the heating until I put insulation above the ceiling. Insulation above the ceiling restricts airflow, so the moisture gets trapped in it.

I know I could have prevented this with damp proof membrane but then the moisture would be condensing right above the plasterboard, with nowhere to go, except back down.

It's best to let nature do its thing, that includes warming the damp air up so it becomes active, and then ventilating the room to let it escape.

At the moment it takes around 1 week to warm up and dry out the room.

The moment the heating goes off, the problem started all over again (New coat of paint, new concrete floor etc etc)

We've ordered a dehumidifier that was due to turn up last week but it hasn't yet.

The walls are almost dry again now, and I have finished with watery products in that room, so hopefully now we won't have any more problems with evaporating damp.

Once this room is done, I'll move onto her hallway and then we can continue on the other side of the house, without any of this hindrance of nagging granny wanting to live in a air tight box with the heating turned up to 25C and no way for the moisture generated to escape.