In article ,
"BillP" writes:

Remove the mortar with a plugging chisel or a rotary rake in a drill.

A lip, minimum of 25mm, should be should be inserted into the cleaned out
joint.

The usual method of holding flashing in place is to cut some lead strips
25mm wide. Roll/fold these over to make lead wedges and drive these into the
joint using the plugging chisel. Then re-point the joint, or use Lead Mate

In my 1900 house, tiny wooden wedges were used which I found just
under the surface of the pointing when racking out. When I repointed
and redressed the lead into the brickwork, I used the same wedges,
which hadn't gone the least bit rotten.

--
Andrew Gabriel