Building regs dictate that a gap is left between the fascia and the brickwork. This allows an easy entry point for birds, rodents, bees and other insects.
Soffit mesh is easy to install and solve this problem quickly. Its a simple job but you must make sure to wear gloves (its very sharp) and also provide an appropriate access solution ideally scaffolding.
You should choose a roll of soffit mesh around twice the width of the aperture you are filling. If its narrower than that choose the minimum roll width of 75mm.
Take the roll and cut of the length required or a manageable length (it can be overlapped). You can cut the mesh with a decent set of household scissors. Fold it slightly down the middle to give it a slight curve. You can then push it into the gap, the curve will spring open and grip into place. Although in most instances it will be fairly secure its best to put a fixing where you can, (see example below of a screw into a joist.)
For ends and gaps you can fold the mesh and wedge it into place. If you don't get the shape correct first time just add another piece.
Example fixing into a joist.
For narrow gaps to stop Bees and insects fold the mesh tightly and use a flat headed screwdriver between the mesh to push it into place.
For ends fold extra pieces and wedge into place. You can use more than one piece if required.
Make sure you get all the gaps.