First off I'm so glad to know I'm not the only sad mpg obsessed Defender fanatic. I have a 1990 110 200 Tdi, which has done about 240,000 miles on the same engine and gear box (neither reconditioned). It used to do 500 miles on a tank (30mpg) regularly and then dipped to 450 miles. I've tried lots of variables over the last few years (Kenlowe fan, biodiesel, K&N air filter, Brownchurch on, Brownchurch off) and eventually narrowed the major difference down to tyres - both style and pressures.
I'm running on Goodyear Wrangler 7.50 R16s set at pressures of 1.9bar (front) and 3.3 bar (rear). As soon as I settled on that the mpg has gone back up to almost 32mpg over the last circa 500 mile tank.
I have been sticking to about 60 on the motorway / dual carriageways and not accelerating too hard or using too much engine braking. However, that doesn't make the biggest difference and I'm now a tyre pressure nerd as well as an mpg nerd.
Incidentally, I expect to get a bit better improvement soon. I've just noticed that after 18 years the seams on the fuel tank seem to be weeping a bit. I don't know, they just don't make stuff like they used to!
PS Best ever was nearly 40 mpg on the way home North through France last year. However, I did have - quite literally - the wind behind me. I had to be careful not to drive too fast. The wind was so strong and I was tanking along at such a speed I though I was going to blow the dang thing up!