just thought i would throw my two pence in I work for the worlds largest motor manufacturer in powertrain engineering, I deal with diesel fuel pumps day in and day out, putting it in the very simpliest terms, there is a very good chance that the fuel pump, all injectors, all related pipeing and filter all require replacement, as well as all low pressure pipeing and the fuel tank dropped and completly cleaned! the fact they have told you not to worry about it is crazy, no doubt!
take it back and go legal! they have completley miss diagnosed your car and destroyed any and all powertrain related warranties that this vehicle would have, there is a very good chance your vehicle will also have been added to the AA or RAC missfuel database which is a centrally stored database to ensure no powertrain fuel related compoents are replaced under warranty once a vehicle is miss fueled!
tell em to sort it, end of story!