Disco TD5 missfire

Hi i'm Martin from the Stockport area, I have developed an annoying sporadic missfire on my 2003 disco (facelift) I have put fuel additive in the tank put a new fuel filter on and filled that with fuel additive so can pretty much rule out muck in the system. I have also fitted also a new injector harness and checked the E.C.U for oil residue and it is clean and still this problem persists, the next step is a new air flow meter but am reluctant to fork out £90 if this isn't going to help. Does anyone have any suggestions as to what may be the best step forward. The vehicle has only done 45k so isn't thrashed.