VAT returns online

I have recently taken on a new client company which is vat registered with vat returns being previously submitted online.

I submitted a 64-8 with the vat box ticked and the vat number but although I now have access to self assessment and company tax online with HMRC, I still do not have access to vat online.  The VAT return is due by the end of December. Whats the easiest way to be able to do this ? Can I submit it under the clients govt gateway username and password? If the client does not have this how should I proceed?