I'm not sure if this has been mentioned elsewhere on the forum, so forgive me if I am unecessarily repeating information. I have stumbled upon a way to access clients VAT certificates after they hae been signed up to MTD.
Log into the old governement gateway, select the relevant VAT client and view the submitted returns. At the top of the page there should be an option to view the clients VAT returns since signing them up for MTD. This link will take you to the new ASA page where you should sign in with your ASA log in details.
It will then ask for the VAT number of you client and ,when you continue, for you to confirm the correct client is displayed. Following this (finally) it will give you options of changing the VAT registration details or viewing the VAT certificate.
A somewhat complicated process, but may be useful to some of you.