My mate has a TD5 110 and we get up to 80-85 on the motorway. Mpg not amazing at those speeds, about 25ish iirc, but ive heard tdi's aren't as good after you get past 55-60. No experience with TDIs though.
Hells bells why would you want to do 85mph in a Defender lol
I have a TD5 and a 300tdi, both are CSW's and both will sit at 60-70 mph on the motorway. However 65mph seems to be the best speed for the TD5 and 60mph for the 300tdi.
If you must, the TD5 will sit at 75mph.