Chimney breast render repair

[Al@n]

I was inspecting the loft and the concrete/mortar on the internal chimney breast within the loft has come away/broken

I'm planning on rendering it again, should I use concrete or mortar?

Found these two products on Screwfix

https://www.screwfix.com/p/no-nonsense-mo42-multipurpose-mortar-grey-20kg/319hn

Or

https://www.screwfix.com/p/no-nonsense-c44-concrete-grey-20kg/748hn

Which should I go for to repair the render on the internal chimney breast within the loft?

 

[Nige F]

The mortar

 

[ree]

OP,
Remove all the render from the loft chimney breast - examine the raw brickwork for cracks or bulges.
And then, not necessary but if you have any doubts about the soundness of the brickwork pointing, you could observe while someone smoke tests the flue or flues.
Use the mortar for rendering the chimney breast - or mix your own at 3:1 sand & cement.