It may be a cracked or damaged waste water line.
Have someone to visually inspect the lines, And also the fan has to be working in order to extract the odours so make certain that it works and that there is no blockage in the extract duct.
And make sure that the toilet is properly cleaned and fumigated, Thats all I could think about.
Last edited by flame_bringer; 24th Jul 2012 at 11:26.