Line a baking tray with greaseproof paper. Pour the crispy rice cereal into a large bowl.
Combine the margarine, sweetened condensed milk and toffees in the top of a double boiler. Stirring constantly, cook over lightly simmering water until melted and combined, about 5 minutes.
Use a wooden skewer to dip each marshmallow in the hot toffee mixture. Immediately roll toffee coated marshmallow in the crispy rice cereal. Remove from skewer and place on the greaseproof paper to harden.