In a large non-metallic bowl, melt the butter and sugar together in the microwave. Stir in the syrup using a hot spoon (to prevent sticking). Stir in the oats. Microwave on high for about 3 minutes (depending on the power).
Spread mixture on the bottom of a lightly greased baking dish. Break the chocolate into pieces and melt in the microwave, keeping a close eye that it doesn't burn.
Spread chocolate on top of oat mixture and leave to cool in the refrigerator until set. Cut into squares.