the solution was staring me in the face, Doh.
public function is_mobile($event)
{
$user_agent = strtolower($this->request->server('HTTP_USER_AGENT'));
$browsers = array('240x320', '320x240','blackberry', 'iemobile', 'minimobile', 'mobile', 'opera mini', 'pda', 'phone', 'pocket', 'psp', 'symbian', 't-shark', 'wireless');
foreach ($browsers as $inbrowserarray)
{
if (stripos($user_agent, $inbrowserarray) !== false)
{
$event['style_id'] = 25;
break;
}
elseif ((strpos($user_agent, 'ipad') == true))
{
break;
}
}
}