Season the chicken and spread onto baking sheets and roast in the oven until just cooked but not too coloured.
Steam or boil the sweetcorn until just cooked. Drain and refresh.
To make the white sauce, melt the margarine in a saucepan then add the flour and cook stirring continuously until sandy in colour and texture. Add the milk whisking all the time. When the sauce has thickened, add the mustard, season and cook out.
Stir in the cooked chicken and sweetcorn and simmer until all the ingredients are heated through.
Stir in the chopped parsley and serve in appropriate containers garnished with sprigs of parsley.