How can I fix the turbo on my '02 2.0l diesel zafira???

Hi, I've just got a 2.0l dti zafira. It has done nearly 300,000 miles as it was a taxi before I got it. The problem is, it is totally gutless!!! I had a 1.6 petrol zafira and that had more guts than this.

I think the problem is somthing to do with the turbo as it just feels really slugish when I accelerate.

Can anyone please help as I don't want to spend loads on tests and stuff at a garage that will add loads of un-neaded extras.