Escort 1.8TD Van running rough - PLEASE HELP!!!!!?

Hi,
I have a 99 ford Escort 1.8TD (non intercooler) van. Have owned for about six months. It has recently started to run rough and i don't know why?
When i start the van, there was always a buzzing sound in the engine for about 2 secs whilst waiting for the glow plugs to warm up, and it would start ok. Now, the buzz has gone and it's a pig to start hot or cold. It idles like it's missing, have grey smoke on idle too. It runs down on power and is drinking more fuel.
I know the turbo is ok because the ehaust is blowing on the middle box, but i can hear the turbo spooling up on acceleration through the exhaust blow!.
Have changed glow plugs and checked power is getting to them. Also changed Engine Management coolant sensor and done oil/ filter change. The Buzz appeared to come from a little round black unit on top of the fuel pump - has 3 wires into it. PLEASE HELP????
I can't afford to put it into a garage to find out what's wrong but need to use the old girl every day!!
Hope someone can throw some advice my way!
Thanks in advance,
Nick