I think I can now answer my question more usefully.
I tried Qustodio which looked promising but was too restrictive: once the computer time was used up with time-limited applications, access was completely locked, meaning the kids could not use any non-restricted stuff either. They also got no warning prior to the lock-up. On the plus side, it shares the allowance across devices (Android, iOS and full Windows) and so they couldn't just switch to another device and carry on.
I have reverted to ScreenTime on their android devices. This allows you to block specific apps after a certain time but still allow others and has various other limits for blocking apps during school hours or after lights-out. Unfortunately the time limit is not shared among their apps so they can switch to another device once their time is up and I just have to account for that by reducing the time allowed. ScreenTime plan to add this in the future but don't have a date yet.
On the PC, I have gone with Parental Control 2015, which again kills access to time-restricted applications once time is up but still allows other applications and web-pages. It has web filtering, can monitor multiple accounts and also has a nice feature where you can enforce an x-minute break after y-minutes of use.
Both ScreenTime and PC2015 have web-based management interfaces so I can pop online from work to see what they have been doing and adjust the time I allow them.