My n reg 1996 diesel 300tdi has started to puff out really black smoke for a few seconds then stops when cruising along.samedays it doesnt do it at all.Before it does it there appears to be a different noise coming from the engine

Black smoke is un-burnt fuel as opposed to oil (bluey white colour), does it do it on the overun - ie after lifting off the throttle at cruising speed or does it do it during acceleration? or does it happen whilst maintaining speed and throttle position?

Each of the above would suggest differing problems, but i would start by checking injector pump timing and bleed/return pipes are clear. May not be either from what you have described, but they are the simplest (and cheapest) causes of black smoke in diesels.

it mainly does it when you cruising along so you may be right about the return pipes will check them and let you know .thanks again