Combine sugar and milk in a large stainless steel saucepan. Stir well to dampen the sugar with the milk.
Add butter and condensed milk then stir over a medium heat until mixture comes to a boil. Once mixture is boiling reduce heat so that the mixture bubbles gently. Stir often until mixture turns a deep tan colour.
At this point a teaspoon of mixture dropped into the glass of water will form a soft ball. (If you have a candy thermometer the temperature of the mixture will be between 113 and 116 degrees C). When you pull a wooden spoon through the mixture it will leave a distinct channel.
Once the tablet has thickened slightly it is ready. Vigorously stir the tablet and scrape down any mixture from the side of the saucepan and quickly stir it in. This will encourage crystallisation.
Carefully pour mixture into the prepared tin and leave at room temperature to set. Be very carful it will burn you if it touches your skin.
It should set relatively quickly (1 hour) but will retain a lot of latent heat until at least 3 hours after it is made. Hence be very careful trying the tablet before it is completely cold as you may well suffer a burnt tongue.
Once the tablet is cold cut into small squares or 'tablets' and serve as treats. It stores very well in an airtight container.