A number of options are open to you depending if you require an exposed soffit to the arch or not. If not, simply use a normal steel lintel to suit cavity masonry or timber-frame and have the segmental arch pre-cut by a specialist brick cutter such as Kevington. The bricklayer will simply lay and point the bricks as part of the wall. If a soffit is required, it is best to have the arch pre-cut as slips and bedded into concrete so the unit is self-supporting and acts as the lintel in its own right. The bricklayer simply positions and points the joints. All brick cutters can offer this service and will generally come to site to collect the bricks required; allow about two to four weeks.