I have to backtrack on what I said. Yes, staying in 6th and around 1300rpm gives you ridiculously good mpg (best I got was 65mpg average on 180 mile journey).
but after doing only two journeys like that, each 180 miles, my dpf clogged up ( P2002 code)
Car went into limp mode, couldnt do much with it.
I did eventually fix it up the next day - drove at 3.5k rpm at 50 mph for good 20 mins, it was a nightmare. Barely drove 2 mins after starting the engine, and it would go into limp mode again. Then had to pull up on the hard shoulder, restart the car - had another 2 mins "allowance". After about 10 pull ups, the car eventually stopped going into limp mode, so I could properly thrash it. Averaged 21 mpg on a 40 mile motorway journey!
After half an hour, and about 100 ****** off drivers who now have to clean their cars from my exhaust smoke (😂), the DPF was clean enough and the car drives properly again.
Seems to have more power too actually.
So as you say, the conclusion is, you can get the good fuel economy by driving the 60mph in 6th, get 65mpg, but always remember to then drive at 75 mph in 5th, so the DPF gets cleaned up, you'll get around 48 mpg on that trip, but when you average it all out, its still a respectable 56.5 mpg average at 67.5mph average if you alternate between the driving styles.