Perfect! Many thanks!
//Save current file one directory up
var CurrentFolder = activeDocument.path;
var parentFolder = decodeURI(activeDocument.path.parent);
//$.writeln(parentFolder);
var BMPFile = new File(parentFolder+'/'+activeDocument.name);
BMPsaveOptions = new BMPSaveOptions();
BMPsaveOptions.depth = BMPDepthType.TWENTYFOUR;
BMPsaveOptions.osType = OperatingSystem.WINDOWS;
activeDocument.saveAs(BMPFile, BMPsaveOptions, true,Extension.LOWERCASE);