Using a heavy based saucepan over very low heat, dissolve the sugar into the water while gently stirring. Make sure not to splash sugar up the sides of the pan. If you do, just use a clean pastry brush with a little water to dissolve it.
Once the sugar has dissolved completely, stop stirring, remove the spoon, turn the heat up to medium-low and bring the mixture to a slow boil. Let it boil for roughly 10-15 minutes until the syrup turns a deep amber colour.
Add the butter and stir until melted.
Now add the cream and stir well.
Bring the caramel back to a boil for around 6-7 minutes, stirring every so often until the mixture thickens and is a dark golden brown colour.
Drizzle a little of the caramel over the biscuit base, then place the marshmallows over the top, pressing them quite close together.
Pour the remaining caramel over the top allowing it to drizzle down into the gaps. Place in the fridge for at least 1/2 an hour before proceeding.