How to approach this?

Hi,

I have what I believe to be penetrating damp on the south facing apex of my house and the extension (added in the 80's), the damp itself I am pretty certain has been caused by having no guttering since I moved in and old rendering, leaving rain to just pour down the side of the house.

I've added two photos, the first showing the inside of the kitchen where there is a very large damp patch that has already caused the paint to peel and the second photo showing the wall from the otherside that is in my dining room leading into the kitchen.

I am expecting that this probably won't dry out due to simply how wet it is, should I take it back to brick on the dining room wall and to concrete on the extension ( i believe the extension is made of concrete blocks), allow it to fully dry out and then re-plaster? Or will a moisture evaporator do the trick?

I am something of a newbie on these issues, I got some very poorly done damp work a year ago (dpc on the entire downstairs) and am keen to avoid the same fate this time round so, whether I do it myself or get someone in, I am keen to at least have an idea of the best course of action

Thanks,

any pics of outside? Is that a door, is the frame sealed to the building well. You need to stop the water coming in and then with help of a dehumidifier let it dry. Hacking off may speed the process up a bit.