Put the sugar, syrup, cream of tartar into a heavy pan and heat gently until the sugar has dissolved. Boil without stirring until a little of the mixture forms hard, brittle threads when tested in cold water (154C/309F on a sugar thermometer). Dissolve bicarbonate of soda in the tin of warm water and pour into the toffee which will froth up. Stir well and pour into a shallow greased tin. Break into pieces when cold. Wrap in greaseproof paper and store in an airtight tin.