Mamama - sorry it took me so long.
I am not exactly sure how much you will need, but am guessing at 6-7 eggs worth although it could be a lot more - 18" * 12" is a HUGE cake - I would be tempted to make several smaller ones.
Anyway, for 6 eggs:
375g SR flour
185g plain flour
375g butter
375g caster sugar
Preheat oven to 160 deg C
sift flours together
cream butter and sugar until fluffy
add eggs one at a time with spoonful of flour with each
Add flavouring if you want - grated lemon rind, or vanilla, or 2-3tblspns cocoa + 1 tblspn milk
fold in remaining flour
spoon into tin and make a dip in the centre with the back of a spoon
bake until skewer comes out clean
cooking time depends on the shape - anywhere from 1-1.5 hours
BTW - it will be drier than normal sponge. I make about twice the stated quantity of buttercream and sandwich it in lots of layers.