1. Place the condensed milk in a large, wide saucepan. Add the lemon juice and cook the mixture over a low heat for 10 minutes, stirring all the time, until it has thickened.
2. Add the toffee-flavour syrup and stir well. Continue to cook over a low heat for a further 10 minutes until the mixture has turned a soft toffee colour. Remove from the heat and carefully stir in the butter.
3. Arrange the sliced bananas over the pastry base and cover with the toffee mixture, drizzling some for effect. Chill until firm, then decorate with whipped cream and banana slices if you like.