jQuery Grid Plugin - jqGrid

I have an application that searches mysql using php, and returns JSON data that is then used to populate a custom written grid/table.

I want to replace this with the jqGrid, but having done so - Im having a few problems when applying new criteria to the grid.

The first time I search, it works fine. The url parameter picks up all the details for the jqGrid command (see below). However, if I then change the form field data and reclick the button which performs the search, the data is not updated (I assume because the <div> is already a jqGrid?

function runSearch(str){
    $("#tbSearchResults").jqGrid({
        width: 820,
        height: 'auto',
        url: '/my/data/clip.json.php?' + str,   .............

Is there another command to use to either recreate the entire grid from scratch using the new parameters, or is there something which will just replace the data content using the new information?

Im sure its going to be something simple, but I have spent a day trying to use the built in custom search features (which work in almost the same way as I need our form to work) but they are just not going to cut it - at least not without some really heavy customisation.

I would really like to be able to just use the existing form with the existing code, with the existing JSON.... but need the grid to accept revised criteria.

Please help!

Many many thanks,

Sam