Butter a 1kg (2lb) loaf tin then line it with greaseproof paper.
Cream the margarine & sugar together in a large bowl. Add the beaten eggs, then the flour and mix well.
Add the ginger & banana & mix again.
Spoon the mixture into the tin and smooth the top. Bake in the centre of a pre-heated oven at 160C (fan)/180C/Gas 4, for 40 minutes. Test with a skewer – if it is still soft in the middle, leave in the oven for a further 10 minutes.
Leave the cake to cool completely before removing it from the tin.
Remove the greaseproof paper & store in an airtight tin.