I use a very simple Pomodoro approach to try and keep focussed on a task, and keep track of how long I spend on it.
I set a timer for thirty minutes (I just use the windows 10 built-in timer) and concentrate on the task (eg writing a report) for all that time - no checking emails, websites or other distractions. At the end of the thirty minutes - mark up another half hour spent on the job, get up and have a stretch and then get back for another half hour.
If I really do have to break off for something else half way through, I pause the timer.
Martin