As you can see I had two replies on Monday, which I read, but nothing since.
Am I missing something?
No, you're not missing anything.
You got an alert, visited the thread (and replied), then got a further alert when HLaB replied.
You won't get any further alerts until you go and read the thread - whereafter the very next response on the thread will prompt a new alert.
Cheers,
Shaun