In a heatproof bowl over a pan of simmering water, melt butter and chocolate with syrup.
Crush the biscuits into crumbs by placing in a freezer bag and pressing with a rolling pin. It helps to assist process by scrunching the bagful with your hands first.
When the chocolate mix is melted, add remaining ingredients and stir together to mix thoroughly.
Place in an 8" square cake tin, lined with clingfilm and leave to set in a cool place. Cut in portions when set.