layerUrl = "https://tiles.arcgis.com/tiles/qHLhLQrcvEnxjtPr/arcgis/rest/services/OS_Open_Background_2/MapServer",

layer = new TileLayer(layerUrl, {
    tileServers: ["https://tiles1.arcgis.com/tiles/qHLhLQrcvEnxjtPr/arcgis/rest/services/OS_Open_Background_2/MapServer", "https://tiles2.arcgis.com/tiles/qHLhLQrcvEnxjtPr/arcgis/rest/services/OS_Open_Background_2/MapServer", "https://tiles3.arcgis.com/tiles/qHLhLQrcvEnxjtPr/arcgis/rest/services/OS_Open_Background_2/MapServer", "https://tiles4.arcgis.com/tiles/qHLhLQrcvEnxjtPr/arcgis/rest/services/OS_Open_Background_2/MapServer"]
});
map.add(layer);