Turn on your oven to 180C(350F) and then grease and flour a 600g loaf tin.
Sieve flour, salt and spice into a mixing bowl. Rub in margarine until mixture resembles fine breadcrumbs. Mash bananas and add to mixing bowl.
Place walnuts into a plastic bag and crush them into small pieces using a rolling pin, then place into mixing bowl along with the eggs and caster sugar.
Use an electric beater to mix until the mixture is well-blended and there are as little lumps as possible. Add in sultanas and stir with a wooden spoon.
Pour mixture into tin and bake in the oven for 45mins to an hour(or until loaf is well risen and has a pale golden colour on top). Once cooked leave loaf in tin for 5 mins and then place on a cooling rack.
I like to eat the loaf warm(10 seconds in the microwave if not fresh out of the oven) with a bit of extra margarine on top.