needing to repair chimney!!?

Well rotor, you look like a handyman ,so you have to get all the loose stuff off, you cant go filling up big gaps all at once you have to build it up in stages,depends whether you need to replace bricks.if you do it yourself use a cement render with less cement and more sand and lime, as this wont crack as much when heated and cooled, see someone locally for advice ,it would take me hours of typing to tell you everything but you should be able to do it yourself, it can be a bit tricky and there are tricks to rendering like throwing the render on and building it up from the bottom, also dampening bricks so it sticks better. Good if you know someone who has done a bit of this type of work

talk to a home builder, stone mason type person

in person face to face.

what you are thinking of doing is not only risky, it may well be a fool's errand as concrete can separate from the brick and crack and break off and fall, perhaps konking some poor kid in the yard on thier little head.