According to "The Great Central From the Footplate" (1987) by Robert Robotham & Frank Stratford the express passenger trains could expect the change to be done in four minutes. Frank Stratford, a driver on the former GCR London extension who came up from cleaner at Leicester shed had a snippet to share. When the BR MR took over the shed they discovered that under-18s at Leicester shed were working nightshifts, which was illegal. There had been a waiver granted during WWII but when hostilities ceased and the waiver lapsed the LNER shed at Leicester just ignored it!