Beechy, preferably get a compression check done as low compression = retarded igntion = white smoke from unburnt diesel. The figure for the 300tdi new is 24bar, I think the 200tdi is quite similar. The reason for low compression is normally worn bores, sorry to tell you that, but it could be valve problems which would be cheaper to fix. Check the blow-by on the bores by taking the oil cap off. If it blows off with increasing revs you have a bore problem.