I would be cautious with this, mortar attacks lead. When considering a new installation of lead flashing the lead should be protected with builders paper or other such medium to act as a barrier between the lead and the mortar. It would add time to your repair, but you could remove the lead packing cut the builders paper to fit insert it then repack the lead wedges back in and point.
Admittedly the detereation will take place over a protracted period.
An organisation such as the Lead Sheet Association based in Tonbridge Kent should be able to advise you (01622 872432).
The mortar mix should reflect the existing mix that has been used to bed your existing brick/blockwork, routinely this is 5 to 1 (sand to cement) but could be stronger say 3 to 1. I would suggest making some trial mixes at different ratios letting it dry and then see which one matches the appearance of your existing jointing/pointing. When you have found the right mix don't make it too wet as it will smear on the brick faces or lead sheet and look unsightly, it should be easily workable not sloppy, slightly less spreadable than butter is a good analogy.
Oh and as said above, if it is lime mortar then it'll be different again!
MUTDs