As has been said you can have 3 reduced daily rest periods between 2 weekly rest periods.
Assuming you do not have split daily rest periods any shift over 13 hours means you’re having a reduced daily rest period, even if you have 11 hours rest you cannot fit the whole 11 hours rest into the 24 hour period so it’s a reduced daily rest period.
So unless you have a split daily rest period you cannot do more than a 13 hour shift tomorrow 
edit:
M65Chris:
As I understand, 13 hours and 1 minute counts as a 15. I refuse to take a 9 hour rest.
It may be to your advantage to stop thinking of 15s and consider anything over a 13 hour shift as a reduced daily rest period, there’s nothing in the regulations about 15s only reduced daily rest periods 
M65Chris:
So tomorrow, I start at 7:45. The latest I can work to is 20:45?
Correct.