Importance of plaster repair skills/experience

Andrew, excellent question and great photos. The problem is a bit complicated. I can see from the photos that the water migration is coming from the top, perhaps even above the top of your window. Anytime the plaster separates from a concrete base, it means the moisture is coming through the concrete, most likely from the out of doors in your case. Unfortunately the proper fix would have to be diagnosed and repaired outside at the source of the leak. Since you have no assess to the outside to explore and repair, the only mitigation to the problem from the inside would be to seal the concrete with a hydo-seal product such a Dri-loc sealer. I really think trying to replace the scratch and finish plaster coats will be a waste of time and fail again soon. I would strip away all the plaster to bare concrete in the effected area, making a clean line at the end of the bad plaster. Attempt to dry it as much as possible with a fan or fan forced heater. Something like a small box ceramic heater would be fine. Strip out as much of the caulking from around the window frame as possible. Paint the concrete with two coats of the Dri-loc sealer. Let it cure completely, recaulk the window frame with a Silicon/Acrylic paintable caulk. Paint the sealed surface with a recommended type of paint compatible with the sealer you select. You will not have as nice and smooth a surface as the original plaster, however, since you cannot stop the source of the water, the goal here is to simply stop it from entering your window bay.

I have had a lot of success in the past by using a mixture of PVA Glue and water and then painting the solution over the crumbling area. The glue gets absorbed into the crumbling surface and stabilises it. Then you can refinish with a thin coat of filler or plaster. It saves a lot of work.