[Javascript] capturing the ID with radio buttons

[javascript@xxxxxxxxxx (Bill Marriott)]

Hi Everyone,

I have a project that needs to capture the current ID (filter value) of the
record that the user selects using Radio Buttons and store it in a variable.
The records are presented in a table form like.

     <tr>
        <td width="5%"><input type="radio" value="V" name="RB" ></td>
        <td width="45%">Murray Valley East            </td>
        <td width="30%">MVE</td>
        <td width="15%">NSW</td>
        <td width ="5%" CLASS="invisible" ID="filter" >7</td>
      </tr>
      <tr>
        <td width="5%"><input type="radio" value="V" name="RB" ></td>
        <td width="45%">Murray Valley West            </td>
        <td width="30%">MVW</td>
        <td width="15%">NSW</td>
        <td width ="5%" CLASS="invisible" ID="filter" >8</td>
      </tr>

I don't know how many rows are going to be presented in the table because
they are generated on the fly in an Informix tag.
The form <form name="radioForm"> is wrapped around all the rows.
Can anyone tell me how I might be able to check if a record has been
selected and how to capture the selected filter value from the table?

Thanks in advance

Bill