About 1/2 packet Digestive and/or Ginger biscuits.
2 oz butter
1 tablespoon sugar (optional)
1 tin condensed milk (I usually use Fussell's skimmed)
3 bananas, sliced lengthwise
1 pint whipping cream, whipped
The day before, place unopened can of condensed milk into a pan of water and boil for three hours - don't let it boil dry whatever you do. Allow to cool. Make biscuit base by turning the biscuits into crumbs, melting the butter in the microwave (only about 30 seconds), mixing the two together with the sugar, if using, and pressing down into a flan dish. Leave in the fridge to harden.
Next day, open the can of milk, which will have turned to toffee, and spread it over the biscuit base - which will try and stick to the toffee, so just swoosh it round as best you can. Top this with the sliced bananas, and instantly with the cream (if you hang about, the bananas will go brown and ucky). Refrigerate until wanted.
I have also made this very successfully with peaches for someone who is allergic to bananas; if you use fresh peaches, pour boiling water over them for a minute to enable them to be peeled easily. Drained tinned peaches are fine, too.