From new the car has been achieving around 520 mile on a tank of fuel (mostly motorway driving), then a month or two back we passed the 20,000 mile mark and it seems like we are suddenly doing over 600 miles without any effort.

I normally expect my cars to free themselves up after about 10,000 miles but then again they have mostly been company cars and have been driven a little harder , the Freely is mine so I've treated it with a bit more respect.

680 miles is the best I've achieved but that was on a long couple of days up and down the country with no urgency and sitting behind lorries doing 50MPH. Cant wait to see what happens at 30,000 mile

You get turbo whine then the dmf goes

Were you using same fuel, i.e. supermarket against premium brand?
Fiat Motorhome: 200-300 mile mixed journeys 28mpg all the time supermarket 26. Checked a couple of times over years and always same result. 4 years on still 28 never varies. All FL2's (3) Gone
2011 Mercedes C180 CGI Gone
FL2 GS Auto SD with heated leather, factory tints, alloy spare and a few other bits.

And there's, me thinking that I'm the only one who has found the the mpg is better with branded fuel.
We get nearly 4mpg better at work on vehicles returning 30 + mpg.

Ah, there may be something in that, I have started using Shell rather than Sainsbury!

18/20 k my last 3 freelander have gained mpg,.... Current: Freelander 2 GS, Orkney Grey, Factory fit privacy glass, design pack and Clearview pack. Almond Leather ,rear spoiler,rear mudflaps.my13.

Gone:Freelander 2, GS, "61"

Gone: Freelander 2, GS.e, "59"

Gone: Freelander 2, S, "08"