Jack Of All Blades

I would first send down the wall and use tsd mixed with water wash the wall down leave to dry then use latex bonding primer it has to be bonding primer cause it's the only thing which works and actually covers over the oil based paint and i'll do 2 coats of that and leave to dry.

Outbuildings and garages to be single storey with maximum eaves height of 2.5 metres and maximum overall height of four metres with a dual pitched roof or three metres for any other roof.
maximum height of 2.5 metres in the case of a building, enclosure or container within 2 metres of a boundary of the curtilage of the dwelling or a house.
no more than half the area of land around the “original house”* would be covered by additions or other buildings.

In my own personal experience with this is strip it all back and dry line the walls one its easier 2 it's less mess and 3 you can paint straight on dry lining once the filler is dried. cuts out the hassle.

No there is no solution they messed that right it's all got be squared off and level theres no excuse of a bodge job. reason plaster couldnt stick was proberly because it was to watery. plaster when mixed should look like weetabix texture