Measure the water into a jug, add the dried yeast and mix to a smooth paste. Leave in a warm place for 10 minutes until risen and frothy. Melt the butter and cool.
Sift the flour and salt into a bowl and stir in the sugar. Add the yeast mixture and eggs and beat thoroughly. Beat in the butter and potato until smooth and elastic, then cover the bowl with a clean cloth and leave to rise in a warm place until doubled in size.
Lightly oil a 2½-pint (1.4 litre) ring tin. Chop the glacé cherries and beat into the risen mixture with the mixed peel. Pour the mixture into the ring tin, then cover with oiled polythene and leave to double in size in a warm place. Set the oven at gas mark 7/425°F/220°C and bake for 30–40 minutes until golden brown. Cool on a rack.
To make the syrup: Add 2 oz (50 g) granulated sugar to ¼ pint (150 ml) water and heat gently until dissolved. Simmer for 5 minutes. Remove from the heat and stir in 3 tablespoons of rum. Stand the savarin on a plate and pour over the syrup while warm.
To finish, rub 4 tablespoons of apricot jam through a sieve into a pan, add 1 tablespoon of water and heat. Brush the savarin with this and decorate with glacé fruits and angelica.