No Chris, it is the particle filter in the exhaust which is blocked. The prices quoted are normal for this, and to my knowledge, they are only available from the main dealers. The exchange unit is one which has been washed out.
Barney, I don't know how handy you are with mechanics, but I cleaned mine out myself. Been fine for 8k+ miles since. I took it off and back flushed it out with several kettle-fulls of boiling water, blowing through with compressed air between. A load of white ash, and some soot came out. I then refitted it after leaving it in a warm place overnight to dry out a little. Apart from a bit of steam when I first started it, it was fine. Cost? well how much to boil up 10 kettles of water? Took a couple of hours or so. The most difficult bit was getting the clamps off the joints on the pipe. The filter on yours, 2001, is what looks like the front silencer box, it has a heat shield under it, and a sensor in the middle. This is actually a heat sensor. At the front end, and rear, there are little pipes which go to pressure sensors, which is where the error message is generated from, when they see too much restriction in the filter. You do have to be careful to get these back in the same order. The German diagram is actually for the larter model, where the cat and filter are in one box with a flange joint to separate them. On yours, the cat is right up on the back of the turbo, the box on the exhaust only contains the filter.