Place the chocolate pieces in a small heatproof bowl. Microwave on medium power for 1 minute. Leave to stand for 2 minutes, then microwave again for 30 seconds at a time until melted, stirring frequently to avoid lumps. (Alternatively, melt the chocolate carefully in a small heatproof bowl over a small saucepan of gently simmering water.)
Unwrap the toffees and put them in a polythene bag. Place on a chopping board and tap firmly with a rolling pin until the toffees have broken into small pieces. Tip the pieces into a small saucepan and add the milk. Cook on the lowest possible heat until the toffee has melted (this will take several minutes, depending on the firmness of the toffee). Remove from the heat.
Put the oil in a large saucepan with a tight-fitting lid and heat for 1 minute. Add the popcorn kernels and cover with the lid. Cook until the popping sound stops, then tip the corn out on to a large baking sheet or roasting tin and leave for 5 minutes.
Using a teaspoon, drizzle lines of the toffee sauce over the corn until lightly coated. Drizzle with lines of chocolate in the same way.