Magic Key Lime Pie

'Magic' because it sets without the need for egg yolks and baking, harnessing the power of acid reacting with fat (lots of fat....) to set, like a lemon posset.  There are so many recipes for Key Lime Pie, some using eggs, some with gelatine, some with a meringue topping, but this is my favourite.  It has the added virtue of being suitable for veggies.  Rich and creamy, yes, but also refreshing and tangy with the zest and juice of 6 limes.  A little goes a long way.  Mind you, so does a lot, ha!  To make: crush 200g Hobnob biscuits in a processor, or a plastic bag and a rolling pin.  Stir into 75g melted butter, and press firmly into the base of a loose-bottomed cake or flan tin.  Leave to set in the fridge, while you prepare the filling.  Into a large bowl, put a tin of sweetened condensed milk (you can use the 'Light' version if you want), a medium carton of double cream (about 228ml, usually) and the finely-grated zest of 6 limes.  Stir, and watch the cream turn an amazing green from the zest.  Now, squeeze the juice from the limes (rolling them under your flattened palm or heating them for 5 seconds in the microwave helps release the juice), and stir into the milk/cream mixture.  Like magic (hence the title), the creamy mixture will thicken before your very eyes.  When the juice is well incorporated, turn it onto the chilled biscuit base and spread out evenly.  Lick the bowl.  And spoon.  Dig out a little spoonful from the mix and eat.  Patch up the top again, erasing the evidence. Repeat. Now, leave to set for a good couple of hours or overnight if possible.  Run a knife round the edge and push up the base, to serve.  This is rich, so should serve 8-10 people.  Allegedly.