How can I plug a neighbour's leak into my flat?

Every week a Guardian Money reader submits a question, and it's up to you to help him or her out – a selection of the best answers will appear in Saturday's paper. This week's question I live in a flat and twice in the past year I've been badly flooded by a leak from my upstairs neighbour's washing machine. Each time I've had to pay the excess, and endure lengthy repairs. When I complain he just fobs me off. How can I stop it happening again? What are your thoughts?