Those earlier discussing whether LNER would be "willing" to assist a failed ECT set probably should know that its not exactly LNER's decision. If a service fails on Network Rail infrastructure, Network Rail identify and 'requisition' the required assistance for it...
Regarding all this talk of diversions and hauling 803s, its also worth remembering that at GWR and LNER signing 800/801 is interchangeable, and so its not crazy to think 802/803 would be, particularly looking at the driving training runs now underway. As a result its particularly worth remembering that Hull Trains have 5 802s for 3 daily diagrams (in the full timetable), so for planned works its not impossible to think one or two might be available for hire to ECT, particularly as if, for instance Lincoln diversions are in place, the Hull Trains service is usually reduced to effectively 2 diagrams anyway.