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.

Boards damp but not rotten. Well ventilated attic so I'm hoping no rot will
set in once the leak is fixed. I suppose it's probable that the valley has
been leaking for some time and has only now become serious. Boarded attic
floor is damp but I'm sure will dry out. Main effect below is damp wall
below valley with wallpaper starting to bubble. I'm sure all that will dry
out fine.

The main concern is the valley, and as you say, the boards and nailing. I
just don't have time or money to start re-laying the valley now, so I just
have to hope that fixing the split will stop the water ingress, and then
will have to tackle the wider problem in due course.

As a side note, one small positive for me is that all my DIY efforts over
the years have involved building in a way for me to get access to whatever
it is, and in this case the roof is one thing I can easily get on to through
a velux from the attic. Bit windy today so for once I am going out roped up.
After my coffee break, of course.

Alistair