To make the base, melt the butter in a small pan. Break the biscuits into chunky pieces and put in a food processor. Blend into crumbs then add the melted butter and, with the motor running, blend until thoroughly mixed. Alternatively, put the biscuits in a strong polythene food bag and bash with a rolling pin. Tip into a bowl and stir in the melted butter.
Tip the crumbs into the centre of a deep, 23cm/9in fluted loose-based tart tin or quiche dish and press firmly into the base and sides. (Make sure the biscuit base is evenly distributed, especially where the base meets the sides.) Chill in the fridge for 30 minutes, or until set.
To make the filling, melt the butter in a medium non-stick saucepan and stir in the sugar. Cook over a low heat as the sugar dissolves, stirring constantly until the butter and sugar look smooth and no oil floats to the surface. Add the condensed milk and bring to a gentle simmer, stirring constantly. Cook for three minutes, stirring until the mixture turns a deep, creamy caramel brown.
Pour the toffee gently onto the biscuit base and quickly smooth the surface. Leave to chill for one hour (and up to eight hours) before topping.
Just before serving, lightly whip the cream in a bowl. It should stand in soft peaks, any thicker and it will look over-whipped when mixed with the bananas. Diagonally slice four of the bananas and scatter half of them over the toffee. Fold the rest lightly into the cream and spoon gently on top.
Slice the remaining banana and put in a bowl. Pour the lemon juice over and very gently toss together (this will stop the banana turning brown). Decorate the top with the lemony banana, poking into the cream randomly, and sprinkle the pie with grated chocolate or decorate with chocolate curls. Serve in fairly thin slices as it is very rich.