Crush the digestive biscuits in a clean plastic bag with a rolling pin or wine bottle.
Melt the butter in a saucepan and stir in the crumbs. Press the biscuit mix evenly over the base and sides of a deep 20 cm (8 inch) round springform tin or similar. Put in the refrigerator for 1 hour.
Make the filling. Put the butter and sugar in a saucepan over a low heat. Once the butter has melted, stir in the condensed milk and bring slowly to the boil. Turn down the heat and simmer for 5 minutes, stirring all the time, until the mixture turns a caramel colour. Pour on to the biscuit base and chill in the refrigerator for about 1 hour, until the mixture has set.
Slice the bananas and toss in the lemon juice. Keep a quarter of the bananas for the top and spread the rest over the filling.
Whip the cream until it forms soft peaks and spoon it over the top. Decorate with the rest of the banana slices and sprinkle with the chocolate shavings.