Thanks both.
I wonder if I should complete or stop the painting?
I’m thinking that completing the painting at least means the concrete isn’t exposed and soaking up the rain. I'm thinking if that painting is done right with stabiliser it’s at least stopping the wall being a chalky mess and will help the filler and buy me time to get a builder to do a render.
But not sure how that will respond to filler or sandex (also saw Trupret masonry repair filler) over it - don’t want to make things worse. Also I don’t want to use up 20l of Dulux then have to try to remove it.