Hi,

I have read a couple of threads regarding dormer window leaks.

The only way to deal with these is to go right back to the beginning.

Choose a dry day and take of tiles/cladding to the affeced area, this means taking off the tile baton as well.
Take of pitched tiles running adjacent to the dormer window as well.
before putting it back together check and replace any rotten batons.

Use aluminium soakers up against the dormer on the roof pitch tiles.
Once the pitched roof tiles and soakers are back in place insert 8" lead over the pitched roof tiles and upstand into the dormer window 4".
Fix new baton and felt to the side of the dormer and re-tile.
A lead saddle will be required at the top of the dormer where the dormer window runs into the roof.