I like to start by breaking up the bars of chocolate into pieces, you can also break up the biscuits in advance if you would like. To avoid making too much mess it's best to break in a sandwich bag or tub. I usually just break them straight into the mixture when ready, but then I end up with Oreo hands!
In a heavy based pan add one tin of sweetened condensed milk over a low heat
Heat it through until it starts to thicken, you will be able to feel this, you will not need a thermometer.
Add all the chocolate pieces and keep stirring as the chocolate melts the mixture will start to thicken and cling to the pan.
Add most of the broken / crushed biscuits and stir through. Keep some to put on top of the fudge
Line a baking dish with greaseproof paper
Pour the mixture into the dish and spread evenly, add the remaining crushed biscuits over the top
Leave to chill in the fridge for 3-4 hours or covered on the side overnight. When ready cut into pieces.