2001 ford transit excessive fuel use?

Hi. I used to get around 32mpg on my 2.0 fwd t280 when unloaded. Now I have been carrying 500 kg in the back and my fuel consumption has gone down to around 10mpg. I just used a nearly quarter of a tank (£25 )on a 40 mile trip. Does anyone have any suggestions why it's doing this? I only plod on at 55 and the engine dosent struggle to pull it. I expect worse economy when loaded, but it shouldn't be that bad. Thanks for your replys.