The increase in pay was (mostly) to compensate for the fact that the controllers are still working the same shift pattern that they started with a year ago -- even though they were promised a 6-3, and then a 6-4.
Serco is stuck with the staffing numbers that were submitted in the bid, though those numbers are unrealistic, hence the staff are working longer hours at position, and more days of work in a month, in order for the system to operate. So Serco had to offer more in order to prevent the slow leak of departing controllers from becoming a major flood. A 10% increase in pay is much more affordable than the penalties for breach of contract.
Will that alone be enough? Insh Allah...