Crush the biscuits in a freezer bag with a rolling pin until they are reduced to crumbs. Gently heat some margarine in a saucepan until liquid (or just leave some out at room temperature to soften). Pour crumbs and liquid/soft margarine and mix in with the crumbs until they start to bind together. Transfer the biscuit crumbs to a round dish (a large Pyrex dish is good) and pat with a spoon so that they cover the base. Place in the fridge to set. Put the tin of condensed milk in a pan of boiling water (NB- don't open the tin first!) and allow to boil for 2 hours . You should put a lid on the pan to prevent all the water boiling off. When done, remove the tin and leave to cool a little (you could pour cold water on it to cool the outside). When cool enough to handle, open the tin carefully - the condensed milk will have caramelised and may well shoot out the opening. Pour/spoon the toffee from the tin onto the biscuit base of the Pyrex dish. Place back in the fridge to cool. Slice the bananas and arrange them on top of the toffee. Whip the cream until peaking, then fold on top and smooth out. Dust with chocolate powder for effect. Keep in the fridge until needed, covered with cling-film.
You may want to make the biscuit base with a lip (a bit like a flan or pavlova). Start the condensed milk boiling, then do the base. Have a rest for a while, then pour out the toffee and start on the cream and bananas. Serve cut into wedges in a bowl.