Thanks for all the advice.
I think I might have gone ahead with the tape, against my instincts,
if I hadn't asked here.

Given the time I could waste trying to find a reliable builder I'm
thinking of doing it myself.
Is that realistic? Can anyone recommend a book/web site which describes
the process.

I'm guessing you chissel out the mortar from between the bricks and
push in strips of lead,
folding each one over the bricks and then across the tiles. The bit I'm
not so clear on is
the join between the lead and the tiles. Does the lead just rest on the
tiles? I've been looking
at other roofs from a distance and the lead seems to form a straight
line down the roof. Is that done by cutting each strip of lead to the
right shape?

Also, is the lead just held in by the mortar or should I wedge it with
something before I put the
mortar in? How far in should the lead go?

thanks again,
Thomas