Thanks Lars. Simple when u know how!!
function updateExtent(){
var myMultiPoint = {"geometry":{"points":[[123456,976543],[445355,345345], [3425424,234234]],"spatialReference":7700},"symbol":{"color":[255,255,255,64],
"size":12,"angle":0,"xoffset":0,"yoffset":0,"type":"esriSMS","style":"esriSMSCircle", "outline":{"color":[0,0,0,255],"width":1,"type":"esriSLS","style":"esriSLSSolid"}}};
myMap.graphics.add(new esri.Graphic(myMultiPoint));
myMap.setExtent(esri.graphicsExtent(myMap.graphics.graphics));
}