Thanks for the suggestion. I had previously gone into Task Manager and looked at Services and confirmed that WpcMonSvc was running. I have just done what you suggested, and can confirm that the Parental Controls item shows as running.
I am pretty certain this is a server side issue from Microsoft. It has changed throughout this week - some past days that were previously showing zero usage are showing usage. And today, the app and Family Safety section of online account show that my son has used 27 minutes and 32 minutes respectively. In fact, he has used about 1h45m, and the computer itself knows that and has thrown the warning that he is close to the 2hr limit and it will lock soon.
Absolutely hopeless, the system is broken, no proper way to report to Microsoft. I will be raising it as per the last chat I had, but I have zero confidence that this will do anything. Maybe at some point it will get fixed. Maybe not. Thanks Microsoft, shocking behaviour for such a key bit of software.