HMRC Corporation Tax Return Filing Issue

I have tried repeatedly to use the HMRC's Corporation Tax Return filing tool (the new PDF thing) and for a couple of my clients it gets to the final authorisation page and once I have input my user name and password it thinks for about 5 minutes and then  closes down the PDF file. I'm certain this means that the return has not been submitted. As the online helpdesk it rather helpfully closed at this time of night, is there anybodyout there who can advise if there is anything I am doing wrong?

Thanks in advance for your help

Sir Digby