truckerjon:
I am sure somebody will tell me I’m wrong, but hey!
And you might have guessed it would be me. 
truckerjon:
Now I would disagree with that. I would say that, the shift started on Sunday evening, counted fully as a driving period from week 1, and therefore you would not be able to do more than 9 hours, if you have already done 2 x 10 in that week. Your next fixed week would start 24 hours after you started work on sunday evening.
The fixed week always starts at the same time and has no relation to when you start work on any given day, 00:00 Monday - 24:00 Sunday. I know there is no such time as 24:00 but it is the easiest way to explain it. That is always the same whether you start work on a Sunday, a Monday a Wednesday or any other day…
As has been said you can extend your driving time from 9 to 10 hours twice a week so if his Sunday night shift started at say 19:00 he would not be making use of one of his two allowed extensions until after midnight when a new week has started. People get confused by referring to 10 hours rather than extending the driving limit. 9 hours 1 minute is an extension but it isn’t 10 hours.
The driving period for that shift will not count wholly for week 1, or week 2, driving periods don’t count for particular weeks, just extensions to the normal 9 hour limit count for a particular week. Have a look at what a digital tacho does with the driving time at 00:00 GMT on a Monday if you are driving. It will count the driving done on that shift so far toward the 56 hour limit for the week just ended, week 1, and the driving for the remainder of the shift toward the limit for week 2.