Season the chicken and spread onto a baking sheet and roast in the oven until just cooked but not to coloured
Steam or boil 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, 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 cooked chicken and sweetcorn and simmer until all the ingredients are heated through
Stir in chopped parsley and serve in appropriate containers garnished with sprigs of parsley