Thanks for the advice -but the water wouldn't be dumped into a vertical
standpipe - it would exit the washine machine, go around the room and
enter into the waste pipe underneath the sink. Apart from holding an
amount of water in the waste, what's the downside? What's wrong with
water staying in the waste pipe for a day? It'll be refreshed with new
waste water the next time the washing machine is used.
Thanks

Simon