Got called to a problem where shower tray wasn,t draining away,cleared the blockage but noticed when draining a full wash hand basin water is coming up the shower tray.The set up is shower tray waste pipe runs along the wall to the outside downpipe and a T-piece collects the water from the WHB about halfway along this pipe,the waste pipe does run downwards from the shower tray via the WHB to the outside downpipe,if just running the taps on the WHB the water gurgles at the shower tray waste and customer says it has been like this snce installed,is there any way to resolve this issue ?