preheat oven to 180'C/350'F/Gas mark 4

beat together the margarine and brown sugar in a bowl until light and fluffy. beat in the flour and rolled oats. use your fingertips to bring the mixture together, if necessary.

press the mixture into the base of a shallow 20-cm/8-inch sqaure cake tin.

bake in the preheated oven for 25 minutes, or until just golden and firm. cool in the tin.

place the ingredients for the caramel filling in a saucepan and heat gently, stirring until the sugar has dissolved. bring slowly to the boil over a very low heat, then boil very gently for 3-4 min, stirring constantly, untill thickened

pour the caramel filling over the oat layer in the tin and leave to set

melt dark chocolate and spred it over the caramel. if using the with chocolate, place in a heatproof bowl set over a saucepan of gently simmering water until melted. pipe lines of white chocolate over the dark chocolate, using a cocktail stick, feather the white chocolate into the dark chocolate. leave to set, then cut into slices to serve