Preheat the oven to 160C/325F/Gas Mark 3 (or slightly higher if not an electric fan assisted oven) and line a baking tray (8" x 12" x 1½" or 20cm x 30cm x 3.5cm) with vegetable parchment or waxed paper.
Wash, dry and chop up the cherries to remove the glacé coating (but retaining some as quarter cherries to decorate the top later). If you coat them in some flour it will stop them sinking to the bottom of the cake. Cream the margarine and sugar together thoroughly until light and fluffy (to help the cake rise with the trapped air). Beat in the eggs, one at a time, until smooth. Stir in the flour and then add the cherries.
Spread evenly into the tin and bake in the middle of the oven for about 40 minutes. Allow to cool in the tin.
Add a little water at a time to the icing sugar (frosting) until it has a consistency which coats the back of a metal spoon without running off. Remove the cake from the tin and spread the icing sugar evenly on top. Decorate with quartered cherries.