OFVS - over fascia ventilation system, this can fix down on top of the fascia board and under the tiles, it also incorporates ventilation into the roof space, the eaves felt protector and also the bird comb which stops them from nesting in the fascia/soffit area.
The length is 900mm and approx 300mm deep (slidding under the felt/tiles).
Visit the Kent Building Plastics website for more information on Over Fascia Vent + Bird Comb & Eaves Protector 900mm