It looks like youve got a tiled roof. Go up on the roof, clean the zinc or lead of the valley with a stiff brush, remove any loose mortar, dry the valley with old rags and if possible a hot air gun, patch up any obvious holes with flashing tape ideally warmed up with the hot air gun the coat the valley with Acrypol, starting at the top. later replace missing pointing, 4 to 1 fine sharp sand/ cement with plasticiser and maybe some pva in the water keep the mix dryer than you think, and press well in
JOHN COUGHLAN
December 2006