i wasnt sure what you were on about with "fluid" so i've just had a quick google and aparently this extra tank contals a fuel additive which is injected into the fuel tank or fuel system to help with the filter regeneration.

http://www.autodiagnos.com/fileadmin...e_overview.pdf

i did not realise these systems existed and to me it seams like another waste of time and extra expense. if your car did not have this system and it was just a filter then i could easily say just remove it as diesel emmision are not done in the same way as petrols are, its just a smoke test and any performance loss could easily be mapped out without any problems. by removing your filter you may have some ecu trouble as it may start throwing up codes.

unfortunately i dont know any more about this subject to help you further, sorry. if no more info can be gleemed from the internet on your type of system and if it can be removed you're next port of call should be a diesel specialist as they will have a lot more info on this subject and will more than likely have come accross your problem several times and a least be able to fix it at a more reasonable cost.

one things for sure, modern diesels need to be thrashed to within an inch of their lives every so often!!