Thanks all - yes its sand and cement -
@nod thanks for this - would you just fill with a flexible filler (https://www.toupret.co.uk/professional-filler/product/problem-surfaces/fill-flex or similar)
if so what brand would you recommend
also - any thoughts on expansion joints - that back wall is over 13M wide
if so just in the render or in the wall as well (retrofit)
then we hope to powerwash - and repaint
we did have a Mica scare in the area but we are reasonably confident its not that