Found this one. Although I haven't tried it, it does sound good.
A bit of Orange Liqueur might make it more orange flavored.
3 eggs in an 8 inch pan should make it really moist.
If you decide to try this, please let us know how it turns out. Thanks.

Double Orange Cake
A really light, moist sponge cake that's particularly good on the day it's made.
Ingredients: ( all measurements approximate )
6 oz soft margarine
6 oz caster sugar
6 oz self raising flour
11/2 tspns baking powder
3 eggs
grated rind and juice of 1 large orange

To finish:
2 tbspns apricot jam
icing sugar
finely shredded rind and juice of half an orange


Method:
Pre-heat the oven to 180C/350F/Mark 4. Lightly grease and line the base of an 8 inch deep round cake tin with greased baking parchment.
Put all the ingredients except the demerara into a large bowl and beat well until thoroughly blended.
Turn the mixture into the prepared tin and level the surface.
Bake for 35 minutes until well risen and springy to the touch.
Turn out onto a rack and leave to cool.
Gently warm the apricot jam in a small pan, then brush over the top of the cool cake.
Sift the icing sugar into a bowl and mix in enough orange juice to form a slightly runny icing, then pour it over the top of the cake and gently spread with a palette knife. (Don't worry if it runs down the side of the cake - this adds to the effect).
Leave to set, then decorate with fine shreds of orange rind.