Well as long as the car is used in some long trips and not round the houses,you should not need additive,but we are all face with a new MOT starting May,and so if you have any doubts about your diesel car then I would run the tank down to a quarter and then add Redex or one of the others and then rev the car to clean out any deposits,I would pick a deserted area to do this,then fill up with diesel and this new MOT will need you to turn up for it and before it starts rev the engine for a couple of minutes this will heat up the exhaust gasses and then you should pass,for me gone are the days of leaving the car and then picking up later with the MOT done you need to be there and rev it.