Many thanks for the comprehensive reply. I see what you mean. Thanks for taking the trouble to explain it to me.
I am having a bad year for smoke and bad running. Earlier in the year the Defender failed the ITV (Spanish MOT) on the emmisions test. It turned out that the timing tensioner (300tdi) had failed and the belt had slipped. I had that sorted (new belt etc.) and passed the test, but a couple of months later the running deteriorated so much that it had to be taken by breakdown truck to my local LR expert. It was just before a long holiday, but he checked the injectors, found them coked up, put a good set in and all seemed well. The only problem I then had was white smoke when the engine was cold. He fixed that by tweaking the timing. Now the Defender was running well - no smoke and plenty of power. Then last week it was time for another ITV test - 6 monthly here for 4x4s over 10 years! So, to make sure it would pass the emissions test I added some MOLY "Stop" to the fuel. It passed the test - better than the opacity reading last time, but I now have whitish smoke, with excessive black when working hard. I wish that I hadn't added the MOLY stuff, because it is probably giving confusing symptoms. I am now hoping that by flushing out the tanks (by using up the fuel) and using good quality diesel and an injector cleaner I may get away with not having to switch injectors again. Because the engine is running reasonably I may be able to "drive" the problem away! But, perhaps I should just take a look at an injector to see if they are partially coked?