Sultana cake

Help, my sultana cake has sunk in the middle, what did I do wrong!It looks ok apart from the middle, golden brown and it looks like the sultanas are even through it.

Cut out the middle and use it for pudding ;)

This is the recipe I'm using now:

GLEN'S CUTTING CAKE ~ Mrs Glen Rowling, Nelson

2 Tablespoons custard powder
450 gms sultanas
1 cup sugar
3 eggs
3 cups flour
2 teaspoons baking powder
1/2 teaspoon salt
1 cup cold water
225 gms butter
1/2 teaspoon of 4 different essences (e.g. lemon, orange, vanilla, rum)

Boil custard powder and water, stirring all the time until thick, then cool
Prepare sultanas
Cream butter and sugar and essences, then add eggs one at a time, beating well between additions
Sift the flour, baking powder and salt and coat the cleaned sultanas with part of this
Add remaining flour alternately with cold custard powder mix to the creamed ingredients
Stir in floured fruit
Turn into an 8 inch square lined with greaseproof paper (2 layers)
Bake at 160*C for 1 & 1/2 hours until a skewer in the middle comes out clean
I have found everyone likes this cake - young and old

From: Country Cookery, recipes from Ohau District, School 90th Jubilee edition 1891-1981

From the web:

Some Reasons Why Cakes Sink or Fall In The Middle:

Cakes will sink or fall because:

1). Overbeating – too much air is incorporated into batter.

2). Underbaking - Oven temperature too low and/or too short a baking time.

3). Over or under measurement of liquid or under measurement of flour.

4). Using too small or large a baking pan.

5). Moving or jarring cake before sufficiently baked or opening the oven door before cake sets.

Note: Only open oven door if absolutely needed, one-half to three-quarter's way through baking.

6). The most common error has to do with the oven temperature. Make sure you have an oven thermometer to test your oven for accuracy.

7). Depending on the recipe if you fold in egg whites, if not beaten fully or folded in too harshly, the cake could fall.

8). Even creaming the butter and sugar too much or too little can cause problems.

Thanks for your hints, could be my oven, I dont think the temperature is correct.

Apart from the sinking in the middle, to get sultanas even through cake always toss in a little flour to coat. They won't sink then. Do this with any fruit in cakes but use some of the flour from the measure not extra flour.