The dates i wanted to sort were ‘dd MMM yy’  e.g. 3 JAN 09   , 13 FEB 09 etc.

The examples of other tablesort parsers i found failed on empty sorts so I choose to have empty values as max date values . hence the 

var nd = new date('12/12/9999').gettime();

you will of course change this if this does not match your needs. 

so here it is.

 $.tablesorter.addParser({
            // set a unique id
            id: 'dates',
            is: function(s) {
                // return false so this parser is not auto detected
                return false;
            },
            format: function(s) {
                // split
            // if s is empty then max date
                var nd = new Date("12/12/9999").getTime();
                if (s != "") {
                    var a = s.split(' ');
                    // get month num
                    a[1] = this.getMonth(a[1]);
                    // glue and return a new date
                    var ad = a[1] + '/' + a[0] + '/' + '20' + a[2];
                    nd = new Date(ad).getTime();
                }
                return nd;
            },
            getMonth: function(s) {
                var m = ['jan', 'feb', 'mar', 'apr', 'may', 'jun', 'jul', 'aug', 'sep', 'oct', 'nov', 'dec'];
                var l = m.length;
                for (var i = 0; i < l; i++) {
                    if (m[i] == s.toLowerCase()) {
                        return (i + 1);
                    }
                }
            },
            // set type, either numeric or text
            type: 'numeric'
        });