Preheat the oven to170°C (fan) / 325°F / gas mark 3. Grease a loaf tin (a 2lb tin is ideal - about 21cm x 9cm or so).
Peel the bananas and place 3 into a small bowl. Mash with a fork until smooth. Place the remaining bananas in another bowl and just lightly mash them, leaving lots of large-ish chunks. Set both bowls aside until later.
In a food mixer or large bowl, beat together the margarine and sugar until pale and fluffy. Add the salt and yoghurt and beat again.
Sift the flour and bicarbonate of soda into the bowl and fold in gently until just combined. Stir through the two bowls of banana, and finally chop the nuts roughly and stir into the mixture.
Pour the mixture into your loaf tin, and bake for an hour or until an inserted skewer or knife comes out clean. Leave to cool in the tin. Enjoy straight away with a cup of tea, or keep in an airtight container.