The East Anglia-Scotland motorail has a short but complicated history. It is difficult to trace from the Public timetables, which just gives a map and list of motorail services. It appears to start as a Cambridge-Edinburgh in 1974, when I have the WTT which shows the northbound train as 1S84 2345 SO and southbound as 1E37 2210 SuO.
In 1978 the train served both Harwich and Cambridge at the East Anglian end and switched to Stirling at the Scottish end. I have the WTT which shows the northbound train as 1S56 2210 ThO from Cambridge and 2000 SO from Harwich, and southbound as 1E36 2302 FO to Harwich and 2057 SuO to Cambridge. The following year Cambridge was replaced by Ely, so the northbound train was 1S56 2230 ThO from Ely and 2000 SO from Harwich, and southbound was 1E36 2300 FO to Harwich and 2300 SuO to Ely. Then in 1980 Harwich was dropped and the train ran 2230 ThSO from Ely and 2302 FSuO from Stirling.
In 1981 the northbound end switched to Inverness and the train ran 3 times per week: 2135 MWFO from Ely and 2020 TThO/2135SO from Inverness. As far as I can tell 1982 was the last year when northbound was 1854 WO Ely-Inverness and southbound was 1824 TO Inverness-Ely.
All of the WTTs I have show the trains running via Peterborough, except 1981, when 2020 TThO Inverness-Ely was booked via the Joint Line. So, in the French Drove picture, the train is on its booked route, not a diversion.