Okay, getGridParam() has data and _index which I can use.

Ok solved this one. I had to clear the filter first:

var grid = jQuery(this.gridId);

grid[0].p.search = false;

jQuery.extend(grid[0].p.postData,{filters:''});

before doing: grid.setGridParam({datatype: 'local', 'data': getGridParamData}).trigger('reloadGrid', [{page:1}]);

only then was I able to make loaded data show.

Turns out it was being paginated so the 'missing' data was in another page. Setting rowNum to -1 forces the grid to show all data.