I would remove the lot,,,you don't know where its blown else where and all your window reveals and heads will be bigger which in turn will make it harder to change windows etc,,,also getting a good key is another thing which has to be considered,,,I normally remove the lot and then metal lath the building...especially if its constructed from thermalite (grey) or durox/toplite (white) blocks....

Also not bad idea is to remove blown render,make a levelled patches and fit 20mm polystyrene boards over,they will be fixed mechanically so whole system keep together,a final coat apply modern silicone just 3mm thick render.and doesn't need to be painted because there is choice from more than 300 colours.

You do not mention approx age of the house but I am assuming it is of brick and cavity construction ? Best to remove all existing render then paint with" Render grip "and scratch coat after having first repaired and replaced missing bricks ! Re render with 5:1 ratio sand and cement with waterproof additive used in accordance with manufacturers recommendations . Bucket gauge the mix to ensure consistent sand and cement ratio ! Hope this helps ?