Grease a 20 x 30 cm pan, cover with baking paper then set aside. Whizz biscuits in food processor until finely crumbed. Reserve 40 maltesers then coursley chop the rest. Place butter and condensed milk in a medium size saucepan and stir over a low heat until smooth. Combine the biscuit mixture with the chopped maltesers then add the milk mixture and combine. Press mixture into prepared tin and chill for 30 minutes. Stir milk chocolate and oil in a heat proof bowl over a pot of simmering water until chocolate has melted and is combined with the oil. Spread over biscuit base then top with reserved maltesers. Chill for one hour before cutting into slices.