I did also use the plums which I cut in half, removed the stone, and sliced to lay onto of the defrosted forest fruits mix.
The best crumble recipe is from The Cookery Year by Reader’s Digest, 1974.
12 oz. plain flour
6 oz. cold butter or margarine
4 oz. caster sugar
1 oz. brown sugar
Sift the flour into a mixing bowl, cut up the butter and rub it lightly into the flour with the tips of your fingers. Mix in the caster sugar. Spoon the crumble mixture over the fruit and press it down lightly. Sprinkle the brown sugar on top.
Place the dish on a baking tray and bake at 180°C/350°F/Gas 4 for 45 minutes.