[bjt]

Once again, thanks very much Ralph.
Having both radio buttons selected was part of the problem. The rest was caused by trying to nut this out way too late at night.
One problem was a mis-spelling of the word variant and the other thing I missed was a closing <td>

Here is the working solution:
{foreach from=$v.variants item=o}
<td align="center"><input type="radio" name="additional_values[{$v.fieldid}]" id="additional_values_{$v.fieldid}" value='{$o|escape}'>{$o|escape}</td>
{/foreach}

Kind regards,
Brett