There was generally a "knock down" of power, where Deltic services would drop to a Class 47, Class 47 to a 46, etc. The most common reason for Deltic shortage was too many of them in Doncaster works for repairs. There was a basic Game Plan for all this when there was one Deltic short, two short, etc. Another aspect was that Deltics only carried on from Edinburgh to Aberdeen if there was a full complement of them. I knew someone who lived lineside in Fife who commented that some days one would pass, other days not.
The magazines of the day, Cecil J Allen in Modern Railways and O S Nock in the Railway Magazine, very regularly covered locomotive performance on substituted services. I vaguely recall a particular one, probably late 1970s, when an "intermediate" Up Leeds service, booked for a Class 47 with Mk2C stock, so not air conditioned, had a Class 40 instead. It seemed very late for one to be on an ECML express. Essentially full throttle all the way, and some decidedly spirited air brake stops at stations.
Presumably all ECML crews signed Class 46 (and Class 40), as they seemed very common substitutes. I remember Class 46 also on Cambridge to Kings Cross fasts.