I doubt they have maxed their pilots out already. Sickness is a possability but more likely to be in Winter. Compared their daily operations of over 100 aircraft, I cant see how the Chartering some B757s, B767s etc over a bank holiday weekend is that significant!
Tech Delays etc are common especially on older members of the fleet. We once had I think it was G-EZYC based at BFS who declared emergencies alot as it kept thinking the engines were faulty.
If the GSM 767 was in daily for them I would be inclinded to think they had a more serious problem!