Place the sugar and milk in a large pan over a gentle heat. Stir occasionally until all the sugar has dissolved. Pre-grease a 13x9 inch tray with a generous amount of butter and set aside.
Once all the sugar has dissolved add the butter and allow to melt.
When the butter has melted, add the condensed milk and mix well. Increase the heat and stir continuously while the mixture boils and reaches Soft-Ball stage (120°c) on your thermometer.
Remove from the heat and allow the mixture to settle a little before beating. Beat with a wooden spoon in the pan until the mixture if almost setting. This may take a good bit of beating!
Transfer to your pre-greased tray and spread evenly to the edges/corners.
Leave to set for at least a couple of hours but ideally overnight. If you want neatly cut squares/bars of tablet score your tablet in your desired size/shape about 30 minutes into setting. Alternatively you can simply break the tablet into individual servings once set for a more rustic feel.