An oldie but a goodie. There are lots of recipes online which I'm sure are fab. Not sure where I got mine from originally, but it now exists in my head.
2 packs of mixed frozen berries
1 pack of frozen raspberries
200g caster sugar (or more to taste)
1 loaf of cheap sliced white bread (you won't need it all, but the number depends on the size of your bowl)
Single cream to serve
In a large saucepan, gently heat all the berries with the sugar until the sugar has dissolved. Don't heat too much as the berries will lose all their shape and it'll go a bit jam-like. Taste to see if it needs more sugar - it shouldn't, you do want it to still have some tartness. Remove the crusts from the bread and dip into the liquid which forms around the berries, this way your sides of the pudding will be nice and berry-coloured. Put dipped-side outwards to line your bowl (pudding basin or decent sized mixing bowl). Once fully lined, spoon in the berries and any remaining liquid. Top with a layer of bread. Cover with cling-film and refrigerate until needed. Turn out onto a serving dish and serve with single cream. Serves 8 quite happily.