Cracked rendering letting in dampness

[geordief]

This is in an interior wall beneath a chimney.

I have attempted to repair the leak on the outside but the dampness has persisted.

The plaster itself is damp and this seems to emanate from fairly large deep cracks in the rendering (and stonework behind)

What would be the best way to seal these cracks? Pure cement or maybe some good quality mastic that I could inject into the cavities using a mastic gun?

 

[bobasd]

why not post photos of the interior and the exterior?
is the render on a chimney stack or a wall?
which floor is the damp showing on?