Engine problems ZD30

Have a 2004 D22 with ZD30. Has started running very badly, often but not always no power, idles badly in that engine keeps revving as if I was blipping the throttle, but the rev counter is reading about double the revs that the engine is at. Vehicle is high mileage (400,000) and used for heavy (3 tonne) towing, but has been pretty reliable. A little white smoke at idle when cold, but otherwise ok, and although engine light goes on shortly after starting, there appears to be no water in the filter. I am mechanically minded, but not familiar with diesels or turbos. Where should I start looking? Thanks

if engine light is on then there should be an error code.
get that read or do it yourself there is instructions around here somewhere.

Code 0702 and 0407

Thanks Tweak'e. After a lot of searching and trying I have extracted the codes.

Have got the codes 0702 and 0407 which are for the crankshaft position sensor circuit sensor or ECM failure, and for an incorrect signal from the injection pump control unit. I will check these for any obvious faults, but where should I go from here?

Found a thread with a result for these codes, will check the wiring and earths and look at replacing the crank sensor. Have to put a new fan on anyway as the old one is cracked, so they can be done at the same time.