Is your issue now fixed?
I have a smilar problem, in fact exactly the same, however my set up is just as Hi spec suggests but water is still draining back up into my sink or into the washing machine.
See image here...
Hi,
When i drain my sink, water tends to run down the washing machine waste pipe which is connected just below the sink before the U bend. When i drain the washing machine, the water tends to drain back up into the sink rather than round the u bend and out the drain pipe. I have unscrewed...