Need to make a fabulous apple and blackberry crumble.

[spudcounter]

Thanks to lovely Mumsnetters, my raspberry and lemon cake turned out brilliantly.
Now I need an equally good recipe for a crumble topping. Mine turn out never to be as crunchy as I envisage. What am I doing wrong?

I always use delias

3oz of butter
6oz self raising flour
4oz demerara sugar also sometimes I use 3oz demerara and 1oz of dark brown