function update_giftaid_example(val, multiplier, defval)
{
    var valid = val.match(/^\d+$/) || val.match(/^\d+\.\d*$/);
    val = valid ? val : defval;

    var article = valid ? 'your' : 'a';
    var value = '£' + val;
    var extra = "" + (multiplier * val);

    $('#donation-article').text(article);
    $('#donation-value-feedback').html(value);

    var i = extra.indexOf('.');

    if (i > -1)
    {
        var pennies = extra.substr(i + 1);
        if (pennies.length == 1) extra += '0';
        else if (pennies.length > 2) extra = extra.substr(0, i + 3);
    }

    $('#donation-extra-feedback').html("£" + extra);
}