Grease a square or round shallow tin (approximately 7in (17.5cm) in diameter). Put the biscuits in a deep bowl or a polythene bag and crush them not too finely with a rolling pin or wooden spoon. Melt the butter, sugar and syrup in a saucepan over a low heat, stirring well, until the butter is melted and the sugar has dissolved. Remove from the heat, and stir in the peanut butter. Mix in the biscuits, stir well. Press into the greased tin, and leave in the fridge or a cool place until set (1/2-1 hour according to temperature). Cut into squares or fingers.