Update and resolution
I upgraded the unit to a Pi 3B+ and repurchased the Codecs, but it still had exactly the same issue. Then I found This thread about wired network issues. Deliberately downgrading the connection from 1Gb to 100Mb again had no effect, so I used the wireless capabilities of 3B+ to connect, lo and behold the system works perfectly!
I definitely feel that some kind of issue has been introduced into the wired connection as it used to work fine, so perhaps this should be considered a bug report