Preheat oven to 170º. Grease the pie tin with a little butter.
Melt the butter over a bowl of hot water.
Pulse the biscuits, sugar and cinnamon in the food processor until you get coarse crumbs or smash manually with a rolling pin and combine the ingredients in a large bowl.
Gradually add the melted butter to the biscuit mixture and combine well.
Tip the mixture into the pie tin and press firmly into place to create a solid base.
Put in the oven for 5 minutes to set.
Whisk all of the filling ingredients until well combined.
Pour the filling over the set base, smoothing out to the edges.
Put back in the oven for 15 minutes until the edges have start to set. The pie should still be a bit wobbly at this stage.
Allow to cool completely then put in the fridge for at least 4 hours but ideally overnight,
Whip the cream and spread over the pie. Dust a small amount of cinnamon over the top.
Slice and enjoy!