HMRC offers a helpful online tool that allows agents and taxpayers to check when they can expect a response to a query or request that they have made. The online tool is updated weekly with the latest information.
The tool covers a comprehensive list of taxes including Self Assessment, Corporation Tax, VAT, PAYE, Capital Gains Tax, Inheritance Tax, Stamp Duty Land Tax, and many others.
Agents can also check how long it will take HMRC to respond to various types of requests and queries.
The online tool can be accessed at the following address, and you do not have to be logged in to receive an answer: https://www.tax.service.gov.uk/guidance/Check-when-you-can-expect-a-reply-from-HMRC/start/are-you-an-agent.