I've received an email from HMRC's Customer Relationship and External Stakeholder team this morning - as follows ...
"Customer Migration Issue
As you may be aware, HMRC recently started moving VAT customer records from the legacy VAT mainframe to the newly integrated Enterprise Tax Management Platform (ETMP). This activity has led to an issue for some Agents with non-MTD clients, who have been receiving an incorrect message in the Agent Online Self-Service (AOSS) account.
This happens when an Agent logs to their AOSS, selects VAT, sees the VAT Client list, and selects a client whose record has been unknowingly moved on to ETMP. Agents are receiving an incorrect message informing them that the client has signed up to Making Tax Digital (MTD), even when the client has not yet done so.
This message will direct Agents to log into the Government Gateway (GG) using their Agent Services Account (ASA) credentials. Any Agent whose client has not joined MTD that sees this message should continue through the screens to sign into their ASA (or create an ASA) to file their client's non MTD VAT return.
This issue will affect around 190,000 businesses whose account has been migrated or will be migrated in the coming months. We apologise for any confusion this may cause - we are currently working on a solution to prevent this incorrect message."
Hope this helps anyone currently experiencing this problem!