How should iI fix concrete roof tiles abutting valley?

I have a concrete tiled roof and noted when cleaning the valleys that the cut tiles abutting same are not bed in concrete or in any other way. The gaps left whilst giving the roof plenty of ventilation are proving popular access for birds to get under the tiles to nest. The tiles are redland stonewold Mk1 ( now discontinued) with a triangular shaped profile which have plastic caps fitted where the tiles meet the gutter thereby preventing birds and insects getting under the tiles