SPARQL syntax for casting a variable (with a string in it) to a literal type

cast_string_variable.sparql

PREFIX rdfs:       <http://www.w3.org/2000/01/rdf-schema#>
PREFIX ospc:       <http://data.ordnancesurvey.co.uk/ontology/postcode/>
PREFIX geopos:     <http://www.w3.org/2003/01/geo/wgs84_pos#>
PREFIX geosparql:  <http://www.opengis.net/ont/geosparql>
CONSTRUCT {
  ?postcode geosparql:hasGeometry [
       a geosparql:Geometry ;
       geosparql:asWKT ?wktlit ;
    ]
}
WHERE {
  {
    SELECT (STR(?lat) as ?latstr)
       (STR(?long) as ?longstr) 
       (CONCAT("POINT (", ?latstr, " ", ?longstr, ")") as ?wktstr)
       (STRDT(?wktstr, geosparql:wktLiteral) as ?wktlit) # <- THERE IT IS!
    WHERE 
    {
      GRAPH ?source_postcodes_graph_uri {
          ?postcode a ospc:PostcodeUnit ;
             geopos:lat ?lat ;
             geopos:long ?long .
      }
    } 
  } 
}