Replacing old crappy flashing

Calling all you flashing experts out there ...

I have a number of 30-year old Alcor (or similar Al) flashings to replace. They are engaged in a brick wall against which the (tile) roof abuts and then bent and hammered flat over the tiles, somewhat held in place by lead fixings engaged to the tiles at the bottom edge ..hope I'm making myself clear enough here.

I don't know why they'd have done 'em in alloy, lead is the obvious replacement material to me, maybe someone wiser can shed light on this aspect. I've got the UK LDA pamplet on lead sizing, lengths etc, and I figure on extracting the existing flashing and widening/deepening the slot with a grinder

My 2 questions are about fixing the replacement flashing in place in the mortar courses of the brickwork:
1. How to hold the lead sheet in place in the mortar course at its top, I think I've seen wedges of some sort somewhere, I guess hammered in on top of the flashing thereby jamming it in place between the brick courses;
2. What's the right mortar mix to refill the gap in the course that has been opened by the process?

Thanks guys

Complete Amateur