Hi,
This sounds like a fault with the Exhaust Gas Recirculation Valve. This valve recycles part of the exhaust gasses to help keep the emissions down. If the car is chucking out black smoke, it means it's over-fuelling.
It will start OK when cold because diesels supply extra fuel to aid cold starting (the same as pulling the choke out on a petrol engine), this is why it's hard to start when hot - it's over fuelling.
The fault may be something as simple as a blown fuse or the valve may be u/s. If you are a member of the AA/RAC, etc, give them a call. If not, it's a trip to your friendly neighbourhood garage I'm afraid. The worst thing in the world you can do is to keep running it in that condition.
Phil, July 2008