Ali Mac wrote:

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.

Way too soon?

Perhaps but lead stretches in the heat and does not contract (as harder
metals do) at night. We had some exceptional weather recently to send
it over the edge.

What your problem was, is not what your problem is now.

The water has affected the boards supporting the lead, the nails
holding the board and the attic below before coming into the house. And
the weather doesn't seem to want to clear.

Get in there and take a couple of halogen lights with you and a camera,
then report back.

Have fun.