HMRC do normally respond to submissions within a few seconds, but they can take, at times, anything up to 24 hours to issue a response, especially if the online servers are down for maintenance.

To cancel the polling you need to click the cancel button. This will mean the return has reached the gateway but no response is available. To get the response at a later time you need to click FBI > Transmit and say yes to continue waiting for previous responses, or click FBI in the toolbar at the top and then Reply.

If you try to resubmit the return it is likely it will be rejected as the original return was successfully received, there was just a delay in the response being returned.