Line a pudding basin with day-old white bread, crusts removed, and also cut two circles of bread, one for half way up the basin, another for a lid. Heat 250g blueberries and 250g blackberries with about 125g sugar until just simmering. Cool. Stir 250g raspberries into the cooled fruit. Put just under half the fruit mixture into the lined basin, fit in the smaller bread circle, pour in the rest of the fruit then cover with bread lid. Put a saucer or small plate on top, weight it and leave overnight in the fridge.
Un-mould the pudding (put a plate over it then turn it upside down).
This is wonderful with raspberry sauce: Puree raspberries then sieve them and stir in a little sugar and lemon juice. You can use the sauce to coat parts of the bread that the juice has not reached. Serve with cream.