Slice Swiss roll into slices about 3-4 cm thick. Slice the bananas (put some slices aside for the custard).
Place a layer of Swiss roll into a large bowl. Put a layer of bananas on top of the sponge. Sprinkle the coconut on top (do not use all just enough to cover). Repeat layers until almost near top of bowl.
Make up the custard and let it cool a bit, then add remaining banana, stirring in well.
Pour custard over the contents of the bowl and allow to fully cool, then sprinkle over the rest of the coconut and store in fridge till ready to serve.