It must be a huge problem for TUI when a 767 or s/h 787 has issues. It’s hard enough to lease that sort of capacity in on short notice and as pointed out in a post above theres next to no slack in the TUI fleet currently so it’s not like they can do what Jet2 did last week when the air tanker A330 went tech and they split the following days flight into two (757 & 737) to avoid delays.