Place the dates, bicarbonate of soda, margarine and sugar in a large bowl and pour over the boiling water. Mix to break up the margarine and then set aside until the boiling water has cooled.
Meanwhile, line a 2lb loaf tin with greaseproof paper and pre-heat your oven to 170°c (150°c for fan asssisted ovens or Gas Mark 3).
Once the mixture has cooled, add the egg and walnuts, mixing through to combine. Sift in the flour and mix to create a batter.
Transfer the batter to your pre-lined tin and bake in your pre-heated oven for 1 hour - 1hour 15 minutes. Keep an eye on your cake toward the end of baking and if you think it's looks done on top, cover with a sheet of tin foil - this will stop the top from burning while the inside continues to bake.
Remove from the oven and allow to cool in the tin. Serve straight away or for a better taste, keep the loaf in an air-tight container for a couple of days before serving.