This is caused either by excess fuel or insufficient air, you will usually get a little black smoke at full throttle but if its leaving a cloud there is obviously something wrong. If the performance is normal it could be that the pump is overfuelng, if its a Bosch pump its quite easy to adjust but any reduction in fuel will give a corresponding reduction in performance.

Has the air filter been changed? That's the first thing to look at, then check that all the air intake and turbo hoses have not collapsed or been squashed.

Normally, I'd concur with the 'lack of air'/low fuel pressure diagnosis-they're the classic causes of diesel smoke. But the fact that it occured after a cam belt change is interesting. Trouble is, the symptoms are contradictory. Advancing the timing will tend to bump up the performance and fuel economy, at the expense of more diesel clatter. Retarding timing tends to quieten combustion, but has a deleterious effect on smoke and efficiency. You've got more clatter, and smoke, so the timing appears to be advanced and retarded simultaneously! Did you change it yourself, (if you don't mind me asking)? I'm not accusing you of being incompetent, the injection timing might have been altered before. A 2 degree difference in SOI would make a big difference.