2004 merc 208cdi starting problem?

no problems since new had full service berfore travelling 3500miles in france 2 weeks later starting problem garage replaced fuel filter pipes air in system etc£200 + lasted one week same problem back to garage another £200+ 2 weeks later same problem this time advised 2 faulty injectors had all 4 replaced fuel pressures checked etc £1200! 2 weekslater engine cut out at traffic lights twice took to garage took it test drive no problem. cheked pipes for leaks all ok. next day traveling from notts to exeter cut out on m5 j 24 air still showing in fuel pipe towed back to home. help any ideas does the garage have to rectify without further cost to us