Was this the time of Midland's expansion into Europe? I'd left by then. Maybe they were short of aircraft.
The 1-11 certainly appeared on MME from time to time. As did the leased 737-200, EI-BTR. And both Leeds and Teesside had five rotations a day. Teesside certainly contributed more revenue than Leeds.
The general (unpublished) policy was to put the "nines" on the prestigious trunk routes where there was competition. The Leeds and Teesside were captive market "cash cows", so had to make do with whatever was available. Hence the 1-11 and the leased "73".
So "our" DC9 often got stolen after the first morning (high-revenue) rotation, positioned to LHR/BFS/wherever if a "9" had gone tech., and we had to wait for a Viscount or spare "9" to position to Teesside to do the (much-delayed) mid-morning flight, which happened to be chock-full of interliners. And caused us much work in re-routing and placating the passengers.
Sir Michael was actually more of an "O'Leary" than was generally acknowledged. A cunning entrepreneur, or exploiter of captive markets? Discuss.
And us poor ground staff were the front-end of this juggling of aircraft and disruption. The regular punters could see through this cleverness more than "Ops" probably realised.
Another twist to this policy was to put the -30 series on routes like BFS, just to sabre-rattle against BA, even though the -10/-15s we had were bursting at the seams.
r