Renault Kangoo black smoke on aceleration

I recently acquired a Renault Kangoo Diesel Van. The van runs fine on tick over and starts well but gives out plumes of black smoke on acceleration. I've done the obvious

New air and fuel filter
New oil and filter, oil is at correct level
ran it up the motorway to clear it but made no difference
My mechanic checked the injectors, one was a little dirty so he ultrasonically cleaned it. Still no change so he suspected the EGR valve. To be honest that was pretty clean but having nothing else to go on he put on a new one. Still no change and engine smokes just the same. One thing he did mention was that the diagnostic tool was showing a fault with throttle sensor. Although theres no problem with the throttle, could a fault be causing a wrong mix of fuel. It does have an oil leek at the back of the engine cant see if this is close to the turbo. Any help would be much appreceated

Mike