declare namespace gml = "http://www.opengis.net/gml";
      spatial:buffer(
        <gml:Polygon srsName="osgb:BNG" xmlns:gml='http://www.opengis.net/gml'>
          <gml:outerBoundaryIs>
	    <gml:LinearRing>
	      <gml:coordinates>
	        278200,187600 278400,187600 278400,188000 278200,188000 278200,187600
	      </gml:coordinates>
	    </gml:LinearRing>
          </gml:outerBoundaryIs>
       </gml:Polygon>,
       500
      )