Homemade scottish tablet

1. 1.Using a fairly large pan, heat the water, melt the butter into the hot water. Before the water reaches the boil, add and thoroughly disolve the sugar. stir often
2.bring the water to the boil and then allow to simmer. When simmering, add the condensed milk. the mixture will be a very light, creamy colour
3.Stirring (very) often, allow the mixture to thicken until it is a deep caramel colour, the texture/consistency of the mixture will change quickly after around 20mins, maybe as long as 30mins from a watery simmer to a thicker bubbling. (the colour may also darken quickly at this point). This is where a large pan is handy as the mixture tends to rise a bit when simmering.
4.to test the mixture, drop a little onto a cold side plate, it should not run and should be thick and sticky after 30 seconds.
5.remove from heat and beat mixture for a good 10 mins. pour mixture into a shallow sided tray (10"x6") there should be enough mixture for it to be about 1cm thick. Allow to cool for 30mins, even in the freezer. slice and enjoy. (If the mixture does not set, you can bring it back to a simmer and give it a further 5 mins or so)