Excessive Fuel Consumption 2005 Prado Turbo Diesel Options

[Dieso]

Hello everyone,
I am hoping if anyone could shed some light on the excessive fuel consumption I am experiencing on my Prado turbo Diesel.

I bought the car last year, brand new. Up the Christmas I was getting reasonable fuel economy. About 10 Litres per 100 km on the highway and 11 litres per 100km in the city.

My fuel economy then jumped abruptly for no apparent reason, 12.5 L per 100 km on the Highway and 13.5-14 around town.
The toyota dearer tried to fix the problem by sending the vehicle to their diesel repairer for a few days. The repairer ran diagnostic tests but could not find anything wrong. However, I am still getting poor fuel economy.

A friend noticed that when I started the car, there was some black smoke. I don't know if this is normal considering the car has only done 13000km. Perhaps this smoke may be the key to the problem.

Could anyone suggest what may be causing this problem and what I should do to fix it.
In appreciation
Dieso (IMG:style_emoticons/default/00000732.gif)