Validation - cant set rules

Validation - cant set rules

I have tried this so many ways that I am totally lost.

I have a form with a field that is only visible when an radio button is selected so I have been trying to set a conditional validation.

I have reduced this to simple try and make the field required as follows:

<script>
$(document).ready(function(){
    $("#formId").validate({
        rules:{
            fieldid:{
                required: false
            }
        }
    });
});
</script>

This has no effect. Other fields on the form have required set using the class method and these work fine.

As a test I have trried to extract the rules back from the form using  $("#formid").rules(); but this wonders of in the void never to return.

I am using jquery 1.4.2 and version 1.7 of the validation plug in. Browser is Firefox 3.5.9 on SUse Linux with Firebug and Tamperdata add-ons.

Any suggestions to which stupid error I am making very much appropriated, it must be a stupid error because they are the ones it takes longest to find!

Many thanks.