Preheat the oven to 180C/350F/Gas mark 4.
Put all the stale bread etc into a food processor with a paddle or whisk attachment and switch on to a slow speed until crumbed but still quite rough. Crumble by hand if you do not have a food processor.
Transfer to a bowl and add the custard, sultanas and rum and mix to a creamy stodge. Line a deep tray with lightly buttered baking paper and pile in the mixture, but don't smooth it too much on top, since you want it to be nice and crisp, with uneven peaks.
Bake for 35-45 minutes until crisp and golden on top. Cut into chunks and make the sauce.