Tyres and fuel economy

OK, i'm up for buying some new tyres. i currently still have the general grabber standard tyres on my vehicle. i want to get some more aggressive looking tyres, but only go off road 20% of the time. i do a lot of road miles a week ~250 or so.
i had pretty much settled on some Cooper ST's as i have heard good things about them.
The problem is BFG have launched the new mud terrains and i have been told they are very good (true?). Quieter on road than the old ones, look good, and most importantly look like they have a better compound than the Coopers, which will lead to a longer life (so more cost effective).

What i want to ask is-
now everyone knows big knobbly tyres will lead to worse fuel economy, but i have been told that with a Defender you will hardly notice the difference because it has so much else eating through the fuel (bad aerodynamics, weight, etc etc). Is this true???? if i go for the muds (with longer distance life assumed) will it not make any difference over the less aggressive STs..................
Any advice, big guesses, true facts are welcome
Cheers!

My 200tdi 90 does 250 miles on a tank. It has 35 x 12.5 tyres.

Just realised that my speedo is uncorrected so will be doing more the 250 miles

my td5 110 is much more thirsty on my special tracks than it was on the worn bfg m/t's my landy came with. I rekon I'm about 3-5 mpg down.