Depends how solid the mortar is.
Theres a tool https://www.kennards.com.au/saw-mortar.html for cutting through mortar, but will make a bit of a mess.

Cant edit my comment yet but i meant flashing not sarking.

As Ashington Homes said, commons getting rendered = brickie rushing through the works.
Render should fill in most gaps, but that corner looks atrocious. Hopefully renderer is good haha.
I would be…