2003 Vauxhall Zafira Car starts ffrom cold but won't start

Hi,

I have a Vauxhall Zafira 2.0 diesel.

It starts on the button from cold but after a drive and turning off when I go to start it the second time it just won't catch.

If I crank it for a while black smoke starts coming out the exhaust. To get it started I have to be patient and wait for 2-3 minutes then it usually starts first time again.

It's starting to get worse now though and I might be sitting around a long time waiting for it to start.

Any ideas? Many thanks