Rat Catcher and Garry - thats interesting in itself. I have a Disco series II 02 model manual. I get on petrol (comparing apples, because I normally run LPG) 14.5 litres per 100 k in and out to town - combination of peak hour and non. On the highway keeping her to 90kph (which is an effort because it seems to like 110) I get much better. I have friends however that cannot get that - one with a manual V8 like mine and many with auto V8 discos - but they use more cause of the auto.
To get 15l/100k out of a 100 series (knowing quite a few blokes with them - and my brother services them) must be highway cruising with very light throttle and no overtaking.
Cheers
PS. My 02 Disco handles my Seafarer Vagabond fully laden with fuel and gear very nicely. Its capacity is 3.5 tonns. Being a manual if I have to start of very steep hills, rather than give the clutch a hard time (never glazed it yet though) I sometimes slip her into low range (which is very low in a LR) and then change to high on the fly. My 02 tows better than my 95 Disco - better brakes and a little heavier; but funnily enough my 95 has a 4000kg tow capacity (owners manual).
Cheers