Replacing (chimney) flashing.

I have redressed leadwork into brick walls a number of times (in different places;-), and that's quite easy if you know how to do pointing. Can't your leadwork be fitted back in place?

You want a plugging chisel to clear out the gap, not an angle grinder. Angle grinder will easily damage brick edges which then limits the brick life. (An experienced roofer might get away with it, but you might not unless you are experienced in controlling angle griders.)

I created lead plugs to hold the lead in place, but metal clips are available too.

Dressing new lead is harder, and I got a roofer to do that.

There was always a strange blob of filler at the base of a wall in my hall, when I demolished that wall, there trapped inside was a plugging chisel, obviously the builders had dropped it down a gap, tried and failed to get it out by making a hole in the wall, then clumsily filled the hole ...

That's the fun bit. ;-) If access is easy.