Someone who works on timetables would have known at least a year ago as they work 2 years or more in advance so the manpower issues would have been known then. It's not like it was a surprise to them, of course once you sack all the experienced timetablers and planners and put kids in there as...
Ask yourself why in 1988 under BR that the original Thameslink came in with all drivers fully trained and guards as they were needed then as well as all the drivers knowing the new trains needed for it. There were no delays or excuses needed as there were proper railwaymen who needed what was...