+1 for Leadmate if the gap is thin.
There is a lead products association web site somewhere...
Did you wedge it in the traditional style, with rolled-up strips of lead hammered in on top of the flashing??
I use a stiff (heavy on the cement) mix, with some coarse sand/very fine gravel added. The coarse sand gives it better weather resistance -- you're supposed to do that with roof ridges and stacks. If it's not very exposed ordinary mortar may do, but it's only gap filling - the lead wedges are actually holding the flashing in place.
If, like me you use an angle grinder to open the slot, instead of a bolster or thin chisel, it's probably wise to brush it out well with a paintbrush and paint it with PVA before re-pointing. The PVA helps the cement bond to the brickwork, and ties up the dust so that it doesn't make the mortar loose. I use a spray bottle usually, as it's quick and gets right to the back. I believe PVA makes it a bit more frostproof, too, while it's going off.
I've done our stacks' step flashings that way (40ft up, house on a fairly exposed hill), and the older one is still fine, 16 years on.
HTH,
E.
