I know you have reached the end of your tether with this but it is worth playing the system. If you have a fleet engineer get him involved, if this thing genuinely needs five injectors then there is something causing them to fail which needs investigating; regardless of whether it ends up being the quality of your fuel. On another level what he should be doing is telling this dealership that he wants “the factory” involved in this TODAY, with a Merc or Detroit Diesel field service engineer turning up and reporting directly to him about how Mercedes plan to proceed and what their knowledge is of similar events. It doesn’t matter if Turners have any Mercs on order at present or not, any manufacturer will be keen to avoid being told by a company of your size: “this failure is the last straw, never darken our door again because we shall never, repeat never buy your product again.” If you FE doesn’t agree with taking this line then he is no good at his job -… taking too many freebies.