I had a similar problem when I bought my 2001 2.2 HDI about 4 years ago. A few weeks after I got the car I got the diesel additive low message. I phoned my local main dealer, Pentons of Christchurch, and was told that I might need to replace the filter and was given an estimate of £600. When I booked the car in the mechanic found that the rear footwell appeared to have flooded at some time and the ECU for the additive was a right off. They quoted me around £1200 pounds to fix everything which would have doubled the cost of the car so I started looking on line. I found the additive and a new filter at reasonable prices, about £200 and after a search on breakeryard.com I was able to get a second-hand ECU. A local garage in Poole, Essanjay Citroen, agreed to fit the parts for me for £200. Unfortunately, the weekend before the car was due to be repaired, the cam belt snapped and broke a load of valve rockers. In the end Essanjay repaired the lot for £800 and everything was fine. What amazes me is that the system could tell me the additive level was low given the state of the ECU.