Client made a few errors on the initial CJRS claim (a slight misunderstanding of the rules) which meant they underclaimed. They contacted HMRC who confirmed, as we know, that it cannot be amended through the next claim. However they went on to explain how to make an error claim, sent a template, called back exactly when they said they would and all in all were very proactive and helpful so hats off to them.
The bizarre bit was that they insisted on taking details of 15 employees over the phone but the rest had to be submitted on an Excel template. They wouldn't accept the whole lot on the template. Bit weird.