Fiesta van runs like a dog

Hi,

I have an S reg 1998 Diesel van. It was working fine recently, but started to play up this morning. When standing idle, the engine ticks over fine, but when starting to move it picks up very slowly and sluggishly.

When I turn the engine off and back on dark blue smoke comes out the back and i hear a knocking noise coming from the clutch housing.

Any ideas much appreciated.

Thanks