is there a specific type of trap that will prevent bathwater finding its way back along the shower waste or is the seal created by the water in the shower trap sufficient. the trap that was there was a low level bath trap with anti-vac - could i replace with a basic bottle trap or with the pressure created by the head of bath water push the trap out
If you have only a slight fall on your waste pipes.
you could insert a inch and a half Mcalpine non return waste valve, to combat waste water from bath being forced up into shower tray.
they are available from plumbers merchants or even B and Q and Wickes.