EWS "Skodas" are not spot-hired for a few hours, but for a few days or weeks at a time. There would be no question of a loco working a couple of Norwich's and then being back to Euston in time for the sleeper.
Not working on the mainline side, it strikes me that there is a long-standing contract between EWS and NXEA to provide "Skodas" as cover for locos that are away on major exams. After getting locos in maroon and gold or triple grey it was only a matter of time before one in FScR colours turned up (indeed, there have been plenty of rumours of FScR locos on the GEML for ages, but it has always come to naught). What the management think about this is not something that has been made known, though I actually doubt many punters have even noticed and even fewer would care.
O L Leigh