Grease a 20 x 10 x 6.5cm loaf tin with butter, then cut a piece of baking parchment to run along the middle and up the ends of the tin. Set aside.
Break the digestives into small pieces and put in a mixing bowl.
Half fill a pan with water and bring to a gentle simmer. Break the chocolate into small pieces and put in a heatproof bowl. Add the butter and golden syrup, place the bowl over the simmering water and leave for 2 mins to melt. Stir for the next few mins until everything is completely melted together – the mixture should be smooth and glossy. Remove from the heat and leave to cool slightly.
Put the bowl on the table, taking care if you're doing this with the kids, as the mixture will still be hot.
Add the digestives, fruit and nuts to the chocolate mixture, and mix everything together with a wooden spoon.
Dollop the mixture into the prepared tin, pushing it down firmly with the spoon. Cover with clingfilm and chill for at least 3 hours until firm.
When firm, gently lift the cake out of the tin using the parchment strips – you may need to run a knife along the sides to help release it from the tin. Cut into about 30 pieces.