A method of retro-fitting an external insulation system to a building, the method comprising the steps of: mounting a plurality of support elements (16, 18, 20) to an existing external surface of a wall of a building; providing a plurality of surface panels (22, 24, 26, 28, 30, 32, 34), each of which comprises an internal wall-facing surface and an external surface; arranging the surface panels so that they cover at least a substantial part of the existing external surface of the wall; mounting each surface panel to one or more of the support elements, so that cavities are defined between the internal wall-facing surfaces of the panels and the existing external surface of the wall; and supplying flowable insulation material into the cavities; in which the step of mounting the support elements to the external surface comprises: positioning at least one adjustable one-piece spacer between each support element and the surface, the spacer comprising: • a first end adapted to abut the external surface of the wall; • a second end adapted to abut the support element; • a main axis extending in a direction between the first and second ends; • a first mounting face which is at a first distance (a) from the first end of the spacer; and • a second mounting face which is at a second distance (b) from the first end of the spacer, the second distance being greater than the first distance; and adjusting the spacer between a first configuration in which the first mounting face abuts the support element, and a second configuration in which the second mounting face abuts the support element, by rotating the spacer relative to the support element about the main axis, to vary a space between the support element and the surface. mounting a plurality of support elements (16, 18, 20) to an existing external surface of a wall of a building; providing a plurality of surface panels (22, 24, 26, 28, 30, 32, 34), each of which comprises an internal wall-facing surface and an external surface; arranging the surface panels so that they cover at least a substantial part of the existing external surface of the wall; mounting each surface panel to one or more of the support elements, so that cavities are defined between the internal wall-facing surfaces of the panels and the existing external surface of the wall; and supplying flowable insulation material into the cavities; in which the step of mounting the support elements to the external surface comprises: positioning at least one adjustable one-piece spacer between each support element and the surface, the spacer comprising: • a first end adapted to abut the external surface of the wall; • a second end adapted to abut the support element; • a main axis extending in a direction between the first and second ends; • a first mounting face which is at a first distance (a) from the first end of the spacer; and • a second mounting face which is at a second distance (b) from the first end of the spacer, the second distance being greater than the first distance; and adjusting the spacer between a first configuration in which the first mounting face abuts the support element, and a second configuration in which the second mounting face abuts the support element, by rotating the spacer relative to the support element about the main axis, to vary a space between the support element and the surface. positioning at least one adjustable one-piece spacer between each support element and the surface, the spacer comprising: • a first end adapted to abut the external surface of the wall; • a second end adapted to abut the support element; • a main axis extending in a direction between the first and second ends; • a first mounting face which is at a first distance (a) from the first end of the spacer; and • a second mounting face which is at a second distance (b) from the first end of the spacer, the second distance being greater than the first distance; • a first end adapted to abut the external surface of the wall; • a second end adapted to abut the support element; • a main axis extending in a direction between the first and second ends; • a first mounting face which is at a first distance (a) from the first end of the spacer; and • a second mounting face which is at a second distance (b) from the first end of the spacer, the second distance being greater than the first distance; and adjusting the spacer between a first configuration in which the first mounting face abuts the support element, and a second configuration in which the second mounting face abuts the support element, by rotating the spacer relative to the support element about the main axis, to vary a space between the support element and the surface.