Pre heat oven to 160c.
Place rum into a microwave safe container, warm in the microwave for 1 ½ mins - the rum should be just starting to boil.
Add raisins to hot rum and mix to coat thoroughly, then stir periodically until rum is absorbed by raisins (see notes).
Grease a 20cm x 30cm x 5cm high baking dish (8 inch x 12 inch x 2 inch high).
Cut each slice of brioche into 16 even cubes.
Whisk together cream, eggs, sugar and vanilla until smooth.
Combine brioche and raisins, then add brioche mixture to baking dish in an even layer - try to make sure that the raisins are evenly distributed.
Carefully pour cream mixture over brioche, making sure to completely moisten the top layer of bread.
Bake for 40 mins.
Remove from oven, sprinkle demerara sugar evenly over the pudding, bake for a further 20 mins, then serve.