CUT
As a matter of interest, the section of valley that split was half way
down
a continuous 3m run and I've been reading loads of posts recently about
1.5m
being the optimum max length for flashing and so on. On the basis that the
same is true for lead in a valley, there is an example on what can go
wrong.
Roof relaid about 15 years ago (slate, mortared ridge tiles, lead flashing
etc) and is starting to need some TLC, which I think is way to soon.

Alistair
A 3m run of Lead is far to long. The max run of Valley Lead should be no
longer than 1.5m. Im suprised that the roof covering on a 15 year old house
is Slate. Are you sure that it is Slate, by slate I mean natural slate and
not man made cement fibre slate or any of the poly cement type.

Unless youve seen how a slated valley is carried out, youve got a very
specailised job on. One nail in the wrong place can cause water ingress.
I would be very interested in knowing what type of slate is on the roof
before I give any ferther advice, as a professional.

Lead should never be nail down its entire length. Nails should be 25mm or 1"
copper, 2" or 50mm apart 2 rows across the top and one side one third of
the length (500mm or 20") at 150mm or 6" spaced. Lead should be Code 4 or
5. Its best to cut to length and formed into a V on a bench before fixing.