Not as high as a TDi Discovery. I was getting about 28mpg out of mine on average before I lifted it, put bigger tyres on and a huge roof rack. Now I get about 22-24mpg.
However, as much as I rate 300TDi engines, and would definitely have one in a Defender, I'm glad I've got a TD5 Discovery. It comes across as a more refined vehicle and I'm very happy with mine.