Bob, thanks. I now realise I can set parental controls for the device even though my account isn't a child account.
The controls are, however, very crude. I can restrict access to the device to only one time period for a set of days, or block access completely to a group of functions. I was hoping to restrict access to individual apps for varying time periods as can be done with some android apps.
I have activated then usage monitoring functions which will support my self-control.