The invention provides a lamp (5) comprising a lamp body (6) and a mounting unit (8), for fitting through an opening in a substrate. The mounting unit has a front ring (10) for mounting against a front side of the substrate and an arrangement of lugs (12), sliders (20) and brackets (30). The brackets have a collapsed position in which the brackets fit through the opening; and an expanded locked position in which the brackets do not fit through the opening and the brackets are positioned for engaging with a back side of the substrate. The brackets are driven into their positions by a pushing/pulling arrangement rather than by using sprung arrangements. This enables damage to the substrate to be prevented.