My 2003 W211 E320CDI with the straight6 648.961 diesel engine has failed it's MOT on emissions. The average reading was 1.53 against a plate value of 0.6. I changed the air filter, though it didn't look too dirty, and added a bottle of Wynn's "Pre-emission test treatment" to the maximum strength they recommend (30 litres of diesel) and have thrashed it for 100+ miles, making sure it was well warmed up before it's retest. However, it failed the retest with an average only down to 0.85. I get my cars tested at the council depot so they do not offer repairs. The car has just under 100k miles, runs fine, no engine mgmt light etc., is fully serviced and is too good to scrap unless I have no reasonable alternative. What are the next recommended steps?