Line a 20cm (8 inch) square cake tin with aluminium foil and brush lightly with melted butter or oil. Combine the icing sugar, cream, butter and the peppermint cream-filled chocolate, with the glucose, in a medium heavy based pan; stir over medium-low heat until smooth. Stirring constantly, bring the mixture slowly to the boil. Keep stirring while the mixture boils for 5 minutes.
Remove the pan from the heat and use a wooden spoon to beat the mixture until it has cooled, lost its shine and has thickened slightly. Quickly stir the chopped peppermint crisp into the mixture. Pour the mixture into the prepared tin.
Place the white chocolate in a small heatproof bowl and stand it over a pan of barely simmering water, until softened. When soft, stir with a wooden spoon until melted and smooth. Take care not to overheat the chocolate. Drop spoonfuls of the white chocolate randomly onto the surface of the peppermint crisp mixture in the tin. Use the tip of a knife or a skewer to swirl the two mixtures together. Allow to set.
When the fudge is firm, remove from the tin and carefully peel away the aluminium foil. To serve, cut into squares or rectangles. The fudge will keep for up to 3 days, stored in an airtight container. Refrigerate only if the weather is very hot.